Back in September, I postulated that Web apps needed extensibility, but that you can't pull the old plug-in, or embedded scripting language tricks. What's a model for extending GMail?
Jon Udell may have named the solution to the conundrum. Instead of the app being extensible, it can have an architecture of intermediation. Udell is a bit sketchy on the details, but I'm guessing it involves generating protocol responses in structured formats that support transfomation. Also, the protocol can be proxied and manipulated at levels higher than dealing with HTTP requests and responses.
Simon Willison puts two and two together and identifies Greasemonkey as enabling the browser to become an agent of intermediation. With a little help from the server end, essentially well structured documents, your browser can start to do a heck of a lot more than render the results for you. Heck, it can even start talking to other distributed services, allowing you to extend things like del.icio.us without having to wait on jschacter.