I'd imagine that, like a standard 3.5" floppy disk, you can use a Zip disk in either, but it'll have to be formatted for one or the other - i.e. you won't be able to transfer data saved on a PC to a Mac or vice versa...
[broken link removed] would suggest that it is possible to use a PC formatted ZIP in a Mac because the Mac supports the filesystem used on PC's. That's the only 'direction' it works in. You can, it would seem, copy stuff from the Mac onto the PC formatted ZIP and have it read on a PC.