yes FAT32 but theres a 4 GB write per file limit. Mac OS X can only read to NTFS by default but Macfuse + NTFS3g (open source) or Paragon NTFS for Mac (shareware) will add write suport. Windows can read/write to HFS+ with MacDrive (shareware).
I noticed FAT32 is slow on the Mac side. I only use FAT32 for thumb drives. In a perfect world, ZFS would be ready for prime time and Windows and Macs could use that by default.
Just format it as NTFS and get an OS X NTFS read/write support utility. The main one (can't remember the name) is seamless with OS X, so NTFS is used as any other drive.
I noticed FAT32 is slow on the Mac side. I only use FAT32 for thumb drives. In a perfect world, ZFS would be ready for prime time and Windows and Macs could use that by default.