On 01-Jul-08, the state of California made it mandatory to use hands-free technology for cell calls for all drivers 18 and over who want to talk while driving. If you're under 18, the restrictions are even more severe: drivers may not talk on a cell phone through any means, nor may they type instant messages. This under-18 ban strikes me as a good idea, as driving accidents are the leading cause of death for that age group.
This move isn't limited to California, or nearby Washington, which implemented a similar ban the same day: and a number of countries are looking into or planning similar restrictions on using cell phones while you're driving, and 10 states and countries require that cell phones be used with hands-free equipment while driving. (In Washington state, where two TidBITS editors are located, text messaging while driving is explicitly banned; in California, 18-and-up drivers can be pulled over if a police officer decides the driver is distracted and unsafe.)
I live over the hill from Silicon Valley and travel there frequently via a winding two-lane highway. Commuters from both the local university town and Silicon Valley have driven me nuts for years with their horrible driving habits while talking on cell phones. Scariest of all are ladies in big SUVs driving in the mall parking lot.
Once I knew the hands-on ban was on the way, I bought and tried four different options for hands-free iPhone use. I didn't plan on getting four different solutions, but that's how many it took to find one that met my needs. Prices vary from free to $129; you may find a solution I discarded works for you.
Apple iPhone Headset -- The original headset that comes with an iPhone (free with iPhone, or $29 purchased separately) is a good, workable solution. A microphone is embedded in the wire leading to one earbud, about 6 inches (15 cm) down the wire. This square block also contains an integrated multi-purpose press button. When a call comes in, squeezing the button answers the call; squeezing it again at the end of the call hangs up. When you're driving, you don't need to pick up the phone at all - simply pinch the microphone switch. If a call comes in while you're listening to music or a podcast, the audio is paused in favor of your ringtone and then the call itself. The audio resumes automatically when you hang up.
No one I called reported any interference when I was driving the car with the window up. Thanks to a windscreen built into the microphone, they could also hear me over the wind noise with the window down. I find the earbuds to be comfortable (some people do not), and the overall wire length is sufficient to lay the iPhone on the console or car seat.
I don't use the iPhone headset as my main solution (as you'll read below), but because it came with the iPhone and takes up hardly any space, I keep it in my car as a backup.
One flaw with the earbuds, however, is that you typically have both left and right buds in at the same time, which might qualify under the laws of some states and countries as wearing illegal headphones. (See "," 2007-09-13.)
Plantronics Voyager 520 Bluetooth Headset -- The  ($99) fits over one ear and communicates with the iPhone via Bluetooth. Performance was excellent, with good noise cancelation, and setup (pairing with the iPhone) was simple. It even comes with a small desktop charger.
In fact, I loved everything about this headset except for the discomfort of the piece that sits in the ear canal. I must have a weird ear canal layout, because wearing it even for a short drive made me constantly conscious of the headset; there was also enough irritation to make the inside of my ear sore. And, I must admit, I'm bothered by people who walk around with Bluetooth headsets permanently affixed to their ears: you try to ask someone a question only to find they're talking to someone. Leave the headset in the car or office.
Belkin TuneCast Auto -- Belkin's  ($79.99) is a clever one-cable system. One end of the cable plugs into the iPhone (or iPod) and the other end plugs into your car's cigarette lighter to provide power, which also charges the iPhone. An FM radio adapter module sits in the middle. When connected and with your car radio tuned to the FM band, you press the button on the adapter. It searches for a clear FM channel and then indicates the specific channel (for example, 89.7) on a built-in LCD. Select that channel on your car radio, and voila! Your music plays from the iPhone, but more important for our discussion here, if a phone call comes in, you hear the other party through that FM channel on your radio.
But that's all it does. You still have to answer and hang up the iPhone manually, a momentary distraction when driving unless you also use the Apple iPhone earbuds. There's no microphone, so I used the iPhone's built-in mic. Plus, when driving around our hilly urban community or driving some distance along one of the major freeways in the Bay Area, the FM station reception would change at least once every few minutes, requiring the unit to search for a clearer FM channel - at which time you would have to change the radio to that channel.
Admittedly, the TuneCast Auto wasn't designed as a hands-free telephone system, but when I could maintain a constant frequency it served the purpose.
Monster iCarPlay Cassette Adapter for iPod and iPhone -- A similarly unusual but effective approach is to play the iPhone's audio through your car stereo without relying on the FM band. The  ($24.95) is a cassette adapter and cable that plugs into the radio's cassette tape slot. (That is, if your car stereo includes one; many newer cars no longer include a cassette deck, although some have a stereo mini-jack input on the front.) The sound quality was excellent, since it wasn't relying on radio reception, even though a wire runs between the adapter and the iPhone.
To make the setup hands-free, I also bought the, a small microphone ($19.95) that plugs into the iPhone and clips to your shirt or, in my case, a small adhesive hook on the dashboard; the iSoniTalk sits between the iPhone and iCarPlay. The iPhone then stays in the carrying case I use in the console of the car. When I get in, I make one connection to the top of the iPhone and everything is ready to go - no settings, no fiddling, and no distractions at any time. Hearing everything (car radio, satellite radio and iPhone music/podcasts) through the car's speakers is fabulous and cell phone callers have no sense of my unusual setup through the car stereo.
This combination turns out to be my favorite, and the one I use all of the time now as it allows me to handle everything through the car stereo. It's also the cheapest solution of the ones I tried.
Parrot Bluetooth Car Kits -- If you spend a lot of time in the car and want something more sophisticated,  sells a number of Bluetooth hands-free speakerphone kits that clip onto the dashboard or visor. I didn't try any of them, which range in price from $129.99 to $299.99, since the iCarPlay and iSoniTalk combination turned out to be the solution for me.
[Rick Fay is a 22-year Mac user, writer, wireless video networking professional, and serious evaluator of technology. He has also used an iPhone throughout the United States and Mexico since 30-Jun-07.]