Yeah, the shutter closed system notification would work well.

But remember it's a 2-way sync, so even though the n900 has sat doing nothing all day you might have edited some pictures. So you still need to sync over cron aswell.

Linux or windows compatibility would be easy. Linux is trivial - it's just exactly like the N900 end (plus whatever you need to do to get pics wherever you organise them).

Windows supports unison, but I don't know enough about regular scheduled jobs there.

I know it needs refinement. As I say it's proof of concept. I'm also short of time, and only have the N900 a short time before I have to return it, and I've other cool things I want to try and make it do.