Oh, if you don't have an invite yet, I'm sorry to say that I've already given mine out as graduation gifts (one to my Comp Sci roommate from CWRU, Evan Perry, who just graduated with his J.D., and two to former intern and soon to be full-time Microsoftie Matt Manela, who just graduated with his B.S. in Computer Science and B.A. in Math...congratulations to you both!)
If you haven't read anything yet about Popfly, I'd suggest starting with John Montgomery's blog and working your way out from there. Here's a great post where he recaps all of the recent press coverage.
Finally, congratulations to the Popfly team--it's a great site, it's *fun* to use, and I only see it getting better from here. I still want to know what happened to the other block I wrote... :)