Beginning with the first iMacs and progressing through blue and white G3s, PowerBooks, and the Power Macintosh G4, Apple has been quickly dropping floppy drives and legacy technologies like SCSI, ADB, and serial ports from the Macintosh line. Although in general I believe that Apple made the right decision, there's no doubt that the process causes pain for people with older peripherals. A variety of adapters and converters have appeared to provide legacy support for the new Macs, and the reverse has been true as well, with PCI-based USB and FireWire cards allowing new peripherals to work with older Macs.
Reading the Back of the Box -- I've come across an unusual little device from GeeThree.com, a small startup founded by former Apple employees responsible in part for the original PowerBook and PowerPC-based Macs. The $50 Stealth Serial Port provides a serial port to blue & white Power Mac G3s, the Power Mac G4s (both PCI Graphics and AGP Graphics), and the 233 MHz to 333 MHz iMacs (but not the latest slot-loading iMacs and iMac DVs). The Stealth Serial Port uses the internal modem slot in these Macs, replacing the modem if one is present, and substitutes a standard RS-422 8-pin mini-DIN port for the phone jack of the built-in modem. It stands out from the crowd of USB-to-serial adapters because it supports LocalTalk, so you can continue to use older LocalTalk-based LaserWriters, for instance, as well as Apple's free (but somewhat problematic) LocalTalk Bridge.
The Stealth Serial Port works at up to 230.4 Kbps data rates and supports the Comm Toolbox and existing drivers for serial devices, so you don't need new drivers. It also works with externally clocked serial peripherals such as serial printers and MIDI devices, with the exception of devices that require a 9-pin mini-DIN connector which draws power from the ninth pin, such as an old GeoPort modem. Other exceptions include the Apple LaserWriter 310 (which Apple abruptly stopped supporting with Mac OS 8.5, although a fix is available at the page below) and Apple's QuickTake 150 digital camera.
The primary limitation of GeeThree.com's Stealth Serial Port is that it must replace Apple's internal modem, if present, so it's not a great solution if you use that modem (although many people prefer third-party external modems to Apple's internal modems). The Stealth Serial Port works with serial port switches, so you could theoretically attach a multi-port switch to the Stealth Serial Port, then switch between an external modem and other devices such as a PalmPilot cradle or a MIDI device. Installation is also tricky in iMacs, and GeeThree.com recommends having an Apple dealer do the work. Luckily, installation is much easier on Power Macs, which provide convenient flop-open access to the modem slot and other internal components.
Insert Tab A in Slot B -- Installation in my new Power Mac G4 was still slightly picky, since I had to remove a metal box that holds the modem's RJ-11 phone connector, install the Stealth Serial Port in the internal modem slot, screw it down, run a cable underneath the video card, and screw the 8-pin mini-DIN connector to the case. It wasn't hard, but care is warranted, and removing the tiny screws from the little metal box that contained the phone jack required that I hunt down an eyeglass screwdriver. On the software side, I just dropped a Stealth Serial Port Extension into the Extensions folder and rebooted.
Overall, I'm quite pleased with the Stealth Serial Port, since it enabled me to save $100 by not including the internal modem in my build-to-order G4 order, and use an external modem I have lying around for the few occasions I need modem connectivity. I've also used it to connect a Palm V cradle to the Mac to save more money, since Palm sells the Macintosh Serial Adapter for about $12 and the PalmConnect USB Kit for about $48. The few times I've had a chance to use the Stealth Serial Port so far, it's worked flawlessly.
(If you're wondering where GeeThree.com got it's name, the founder was Bruce Gee, who had left Apple and initially formed the company as a "hobby-business." Since the Power Mac G3 had just come out, he couldn't resist the play on words. Then he decided to continue the joke by adding ".com" to the name just so he could say that he'd started a "dot com" company. Along with that puckishness, Bruce found himself on the receiving end of a naming slip-up shortly thereafter. He wanted to register the domain stealthserial.com, but since he was doing it over the phone, he was surprised when he ended up with stealthcereal.com. Amusingly, that domain is still active.)
The GeeThree Stealth Serial Port may have to sneak up on you, but it's an elegant solution to the problem of using legacy serial devices with many newer Macs. And you don't have to worry about buying one that matches the color theme of your Mac's case.