I think Macs can read FAT32 no problem, but the issue is that with Windows you can only create FAT32 partitions of a maximum size of 32GB, which would be a real pain on a large drive.
I know Linux has a read-only NTFS driver, so it's likely that OS X has similar (it's based on FreeBSD Unix), but that's not much cop either as you won't be able to write to it on Mac.
So if you are going to switch platforms, I'd plan on backing up all your data (which you should be doing anyway ;)) and re-formatting the drive when you make the switch.
Muttley