I ran across this video in my bookmarks - this looks like the result of Yahoo acquiring Xoopit. It looks like they've got a platform for building "applications" that users can add into their webmail client. (Here's the developer info )
It's a little like Facebook's games and other apps, and I'd be surprised if that's not what they're competing with. However, it's another good idea from web apps that would work in desktop apps too - why not allow other applications to easily see and work with the data you've got in mail? This kind of app could either be a plug-in add-on to the mail client or a separate app that just accesses the mail data directly. Supporting both would be great.