There are two big controversies brewing in the iPhone world right now, both squarely in AT&T's purview.
The first is that AT&T defaults to sending you detailed information about your phone use, including a printout, arriving through the mail, of every text message you send or receive, as well as every block of data; they do this even if you have unlimited service. The canonical example is a customer whose cost AT&T $10 to send.
This is an idiotic waste of paper ( at nearly 75,000 trees per year), but reportedly customers signing up after 10-Aug-07 will instead receive summary bills that basically just say how much you owe. You can also ask AT&T to switch your account to summary billing or to paperless billing, though  purportedly from an AT&T call center employee claimed that paperless billing would cost $1.99 per month.
But that's not a big deal - AT&T will work it out eventually.
The bigger problem is that, while iPhone data usage is flat-rate in the United States, in every other country it's charged by the byte, and the charges are quite high. Same thing with text messages.
On top of that, many people have their iPhones set to check email automatically every few minutes, something that's not a problem when in the United States, but which generates huge charges when you're in another country.
Consider, for example, that in Canada the charge for outgoing text messages is 50 cents per message, while the data rate is about $1 for every 50K, or $20 per megabyte. Now go to Home > Settings > Usage on the iPhone and multiply your data use, in megabytes, by $20, and contemplate how high your phone bill might be.
There are a few ways you can avoid the high bills that jet-setting iPhone users have been seeing: