Q: I just got a new iPod classic and I know with the older ones you could either use it with a Mac or a PC but not both. Is this still true, or can I use it with both?
A: All of the traditional Click Wheel iPod models such as the iPod classic still present themselves to your computer as external storage devices. This is how iTunes actually manages the content on your iPod, and is the reason why the iPod models are formatted in an OS-specific manner. Windows-formatted iPods use the FAT32 file system, while Mac-formatted iPods use the HFS+ file system. Windows cannot read any HFS+ formatted disk without help, however Mac OS X can read and write to FAT32 drives.
The upshot of all of this is that if you first format your iPod using iTunes on a Windows PC, then you will be able to access it and load content onto it from both Windows and Mac OS X normally, with the only exception being that you will need to return to the Windows PC to update your iPod firmware when necessary, as iTunes on a Mac cannot do this to a Windows-formatted iPod.
On the other hand, if you’ve first formatted your iPod on a Mac, you will not be able to read it on Windows without either doing a full restore on your Windows PC (erasing everything on it and reformatting it for Windows), or purchasing a third-party software application such as MacDrive ($50, http://www.mediafour.com/products/macdrive, trial available) to allow Windows to read Mac-formatted disks.
It should also be mentioned that using a Windows-formatted iPod on a Mac is not a configuration which is supported or tested by Apple. It generally should work without any serious issues, however there have been some minor problems observed in the past with things like album artwork.
Note that the iPhone and iPod touch do not have these platform-specific issues. Although these devices run OS X under the hood (and therefore some variant of an HFS file system), they do not communicate with your computer using “Disk Mode” but rather use an entirely different synchronization protocol that is not OS dependant.