The UI is very nice, well done. I'm not sure how big a market there is for pledges to build software, though. How about expanding the scope? Say I want to organize an event for Facebook Apps developers, but I need help with the expenses. I put up a pledge for, say, $200, and link to it from my blog. If enough people pledge and the cost is covered, then I'll setup the event. Otherwise, I'll know that there is not enough interest. And, of course, software can still be a category for those who need software built.
Well, in fact our system allows for non-software projects quite nicely. But we decided our first focus would be software, mainly because we didn't want to spread ourselves too thin too quickly.
Anyway, your FaceApps Developer Expo sounds like a sweet microPledge use case. Users can withdraw their pledges if a project doesn't finish on time (e.g., you don't get enough money for the event and decide to call it off).
