On a more semantic issue:
The described sync algorithm will work. In most scenarios the abort condition (e.g. all items on a historical feed are known) will also do the job. However this still means that clients need to check the first fh:prev document if they know all entries there - if my understanding is correct.
Is it worthy to think of something to spare clients and servers this lookup? Are the HTTP caching and If-* header mechanisms good enough to save network bandwidth?
An alternate stragety would be to require that fh:prev documents never change once created. Then a client can terminate the sync once it sees a URI it already knows. And most clients would not do more lookups than they are doing now...
//Stefan
