EricTN: only when it's spam 
BHargett, Pcube: That demo was running against live AOL web mail service, over the Venetian wireless, on an HP laptop. The trick is twofold -- first is that with Silverlight 2, the code is compiled and JITed C#, so the data structures in memory are really fast. Second is that they use Silverlight's protected isolated storage to cache things -- similar conceptually to the browser cache, but a lot more flexible and powerful.