As far as I know, and I've looked into this thing extensively since I think it's the most natural and useful feature, sadly there's nothing like what you want. Chronosync is a half baked solutions to sync folder between macs, and of course there's idisk, but nothing like what you are really asking for sadly. A nas could be also helpful as an intermediate device to draw files from, but I mean, really, it's complex the way things are now. Ideally to me everything should sync to the cloud and then to each mac based on preferences, but I am not holding my breath out for this.
I've been wanting this so long, but it's not happening any time soon. So for the time being I am thinking of somehow centralizing media on a nas and then drawing from there, centralizing the most commonly used current files on the cloud in say idisk and dropbox, and using mobileme to sync the rest as I do already.
It's cumbersome though, I get a headache just thinking about it.
But we can't even sync our iphones to two different itunes libraries at the moment (without a hack that is) let alone syncing all devices seamlessly together. I sure hope that N. Carolina center from apple is put into good use, because I am starting to get pissed off with coughing up my annual subscription to mm just for a mere 20gb shared between mail and idisk.
Sd cards, usb flashes, and external hds are always an option, but these are all half measures...