You can hardly have missed the explosion of "Web 2.0" sites, and the excitement around AJAX and in the .Net community, ATLAS.
However, when using the wonderfully simple UpdatePanel from ATLAS, it's actually the markup that's being sent over the wire as part of an asynchronous call.
It's been at the periphery of my consiousness for a while, but I came across a link to Jayrock today that makes me mindfull of optimising the out-of-band communications by using something a bit more clever than markup deltas.
Jayrock is a framework for JSON and JSON-RPC for .NET - it makes exposing services to JSON-RPC as easy as creating an HTTPHandler.
And it's open source too!