[caribiner23]

If you format your iPod for the Mac, it will only be readable on the Mac.

If you format your iPod for Windows, it will be readable on both Mac and PC.

I have both an iBook and an XP machine, with iTunes running on both. Both the iBook and XP can see my iPod, 'cause it's Windows-formatted.

You can use iTunes to get your PC-based music to your iPod-- you just have to reformat your iPod for Windows. You'll have to reload your Mac-based music, but that shouldn't be a huge deal.

Another answer would be to copy the PC-based music to your Mac and import them into your Mac's iTunes app. Then it's all right there.

Good luck!