You can absolutely set up your iPod to work with both a Mac and a PC. The problem is that while PC-formatted drives (FAT32) can generally be used by both Mac and PC, Mac formatted drives (HFS+) can only be read by Macs.
To complicate things, iPods by default will only work with one iTunes
library. Making your iPod capable of syncing to both a Mac and a PC requires formatting the iPod as a PC drive, and tweaking its settings to make it compatible with multiple iTunes libraries. Before doing any of this, be sure you have the contents of your iPod backed up somewhere. Reformatting will completely erase the contents of your iPod.
To see if your iPod is formatted as a PC-compatible drive, just connect your iPod to a PC and see if it is recognized in iTunes. If your iPod pops up in iTunes, then skip to the next step. If your computer prompts you to reformat your iPod, make sure your files are backed up and then hit OK and let iTunes walk you through the steps to reformat your iPod (this should only take a minute).
Next, in order to allow your iPod to sync with multiple libraries, you'll need to make some adjustments in iTunes. With your iPod still connected to your PC, go to your iPod's summary window in iTunes by clicking on your iPod's icon under Devices in the left column. At the bottom of the large summary window, you'll see a section called Options. Under this section, make sure that both the "Enable Disk Use" and "Manually Manage Music and Videos" options have checks next to them (if the disk use box is checked, but grayed out, that's fine). Next, safely eject your iPod and then connect it to your Mac. If all goes well, your iPod should show up in iTunes on both Mac and PC without any problems, although you may need to check off the same Options boxes on your Mac version of iTunes as well.
For more information on all things iPod, check out CNET's iPod Central