Along the lines of AMQP, take a look at www.rabbitmq.com It's a doddle to set up, lightning quick (we regularly run at ~80,000 messages/sec between Europe and the US over the public internet), will run in RAM or on disk and supports quite a few message distribution models, including allowing an ad-hoc number of consumers to connect and receive messages - so each client would just register its own queue update consumer when it was fired up, and when an update is made in a client it could be published out to all.

 

It's worth doing the 6 tutorials so you can see just what it can do - a solution may present itself to you.

Yeah, I've looked at RabbitMQ in the past (and I suspect it's much nicer than Apache's ActiveMQ which is the only AMQP thingum I've used). It has a nice site of friendly client-side libraries, too, for various languages, so it'd probably be what I'd plump for if I went that route.

It only takes care of the messaging part, though; I feel like there should be something out there that builds a layer on top of this and automagically syncs data. Nothing quite seems to fit the bill, though.