Where do you want the Drobo to live? Give it a native file system for where it will spend the most time. Droboshare counts as a linux box. If you’re using a DS, then the file system does not matter to the network clients, they will see an SMB (samba) share, and not the native file system at all.
NTFS-3G will let you access NTFS on linux & MAC
Linux has a (crappy) HFS driver, and for windows, you need to pay.
FAT32 is well supported everywhere, but has lots of limitations.
You will get equal portability with either FAT, NTFS or ext3.
Permission models are all different on all the file systems, and the DS tries to abstract them away as too complicated for the target market. So there isn’t really much hope for portability in that.
If you want to leave the system under linux or XP for a large portion of the time, keep your LUN <= 2 TiB.
win >= Vista support larger LUNS as does OSX.
Generally, the native file system performs best on it’s home turf. (ext3 on linux will run rings around ntfs with the same CPU under linux. Under NT, the difference will likely shrink.)
My environment is entirely linux, so I only run ext3, but ymmv.