Well this tells it all right here. I was shocked about the migration from Mac to PC though!
http://docs.info.apple.com/article.html?artnum=60920#faq8
Question 8: I have both a Mac and a PC. Will my iPod work on both?
Answer: No, 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.
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 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.
Using the iPod for Mac on a PC, or using the iPod for Windows on a Mac, is not supported by Apple.