I just happened to be looking this up yesterday ...
From Apple's iPod FAQs:
I have both a Mac and a PC. Will my iPod work on both?
No, iPod is specially configured for either Mac or PC. iPod for Mac comes in the HFS+ 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.
If you purchased an iPod for Windows, you can easily reformat it to work on your Mac by downloading the iPod for Mac Software Updater on the Apple website. Note that once it is reformatted, it will only work with Macs. You need Mac OS 9.2 or Mac OS X version 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 usless you own an iPod for Windows, which includes the iPod for Windows software updater. This software is only available when you purchase an iPod for Windows.
Using the iPod for Mac on a PC, or using the iPod for Windows on a Mac, is not supported by Apple.
Can I use my existing iPod for Mac on a PC, or my iPod for Windows on a Mac?
Using the iPod for Mac on a PC, or using the iPod for Windows on a Mac, is not supported by Apple.