Switchers who already own iPods for Windows, this is for you. iPod is specially configured for either Mac or PC. iPod for Mac comes in the HFS Plus file format which is optimized for performance and usability in Mac OS X and 9. iPod for Windows comes in the FAT32 file format which is optimized for performance and usability in Windows ME, 2000, and XP. You can easily reformat iPod for Windows to work on your Mac by downloading the iPod for Mac Software Updater on the Apple Web site. Note that once it is reformatted, it will only work with Macs. You need Mac OS 9.2 or Mac OS X 10.1 or later to reformat an iPod for Windows into an iPod for Mac. You cannot reformat an iPod for Mac to work with Windows, but why would you want to? Using the iPod for Mac on a PC, or using the iPod for Windows on a Mac, is not supported by Apple.
(UPDATE: 1:29 pm EST) As pointed out below, Apple’s own Knowledge Base articles conflict and, yes Virginia, it is possible to turn a beautiful, elegant iPod for Macintosh into a clunky, ugly iPod for Windows using the “iPod for Windows 1.2.1 Updater.” More info here.