Last time I tried to implement my own sync solution with rsync I stumbled upon the inability of rsync to track deleted files in two way syncs. If you pass -delete it deletes missing files for both ends regardless if a file is new on one side. I resorted to using unison, which works like a charm four-way between my windows pc, ubuntu box, N900 and macbook. I suppose that compiling it for N9 should be trivial, if the debian binary doesn't work oob.