Right now I don't know much about it but it looks like it might be interesting.

Popfly is a set of online visual tools for building Web pages and mashups (Creator) combined with an online community where you can host, share, rate, comment and even remix creations from other Popfly users (Spaces).

More information will be released at Popfly.ms as the project moves forward, meanwhile check out John Montgomery's blog for a more personal insight.

Update: a bit more information on TechCrunch, and I just got a mail with some more info internally (but no, I'm not telling unless you ping me from a Microsoft email address!)