ext3...

... or ext4 WITHOUT extents.
(since with extents it's no longer backward compatible and can not be used as ext3 fs)
so i think that n900 will mount it as ext3 and other unix-OSs as ext4 (with all improvements like online-defragmentation and checksums in journal)

edit: haven't tested it yet, but you could try it... and if it doesn't work reformat as ext3...
and of course... post your experiences here ^^