OMG! Look at the screecast on the overview page. Wow!
http://www.popfly.com/
It's a Web based mashup program build with Silverlight. The Alpha is on invites only (à la GMail).
This is taken from the FAQ : 
Q-Why did you call it Popfly?
A-Well, left to our own devices we would have called it "Microsoft Visual Mashup Creator Express, May 2007 Community Tech Preview Internet Edition," but instead we asked some folks for help and they suggested some cool names and we all liked Popfly.