You can do incremental backups using tarsnap if you really want, using the --newer-mtime option (tarsnap can do anything that tar can do); but you don't actually want that. Tarsnap automatically does snapshotted backups (I posted some explanation of the difference a few minutes ago in http://news.ycombinator.com/item?id=183213) -- so all you need to do is tell tarsnap that you want to create another backup archive, and it will magically avoid storing multiple copies of the same data.
I guess this brings up a good point. Does tarsnap do incremental backups?