One thing to bear in mind with Unison is that it doesn't (at least last time I checked) handle Mac OS9-style resource forks at all. Rsync (under Tiger at least) is meant to be able to, but I've also read things about it not quite being 100% there yet. I'm presuming that might be an issue, given the OP's mention of iDisk.

Having said that, I can't actually remember the last time I had to deal with a file with a resource fork...