Arq does incremental backups... but does it do incremental restores? i.e. if I have already restored the backup to another computer, can I later fetch just what has changed? This is the only feature missing from duplicity[1] to make it perfect.
No, but it's on my to-do list. It'd be awesome for, for example, restoring from an older Time Machine backup (fast) and then merging in the latest Arq backup.
[1] http://duplicity.nongnu.org/