The state of the art in rich browser-based apps

Via Lambda the Ultimate, I found a great article on how rich client-side web apps are evolving and where things like OddPost and GMail have come from, and a bit of a look at where things are headed.  This is a space that has always fascinated me; maybe I’ll get back into in the near future.  I always liked the idea of a client that did a smart job of fetching and caching data, reducing roundtrips to the server.  It just seemed like the right way to go, but it was fraught with perils.