So, it turns out that OpenAFS doesn't exactly do what I want (and does a lot of stuff I don't want or don't care about).
Unless I find something close to what I'm looking for, I'll probably start work on it myself.
I woke up this morning with a ton of implementation ideas (and a rough plan of how I want to start implementing things). I've got the first few stages thought out.
I want to first make the server daemon and a client app just to test file sending/receiving and some basic revision control stuff (eventually I want both point in time recovery and some kind of branching/merging functionality -- maybe merging only for text files).
After I get that part working, I'll start on the cool client integration. I'll have to write something like the webdav or ftp plugins to get it to look like a filesystem to the finder. I'll probably also have to write some kind of automount daemon.
The key will really be that everything will work locally and just sync up if/when a net connection is available (I'll have to put a pref. somewhere so people can turn off automagic sync. so they don't use too much bandwith over something like gprs).
... I'm excited about it, though. It's something I'll at least find useful, I think others will like it too, and it should get me some attention. (Besides, it'll let me do both network, filesystem, database, and revision control style coding which could be fun).
Now I just need to make some time to do this, and start writing.