On the current iPods, and with the current iTunes, you cannot use the same iPod to tranfer files from a PC and a Mac. It's difficult in itself to make an iPod to work on 2 different Macs as it's really designed to get mp3s from the same Mac everytime.
You can format your iPod to work with either a PC or a Mac, but not both machines.
The new ones may change that, however.