Welcome to the site berwyn
I should say that generally speaking I think the pricing scheme looks quite reasonable to me. I particularly like the idea that the open source licensed software is free, and i like how you seem to have thought out the issues involving canceled projects, etc.
With such services and sites my focus is always on the "incentives" of the system -- do the rules of a system motivate people to behave well or behave badly? Does it motivate them to create stuff that helps everyone involved? Or game the system to take as much as they can and give as little as possible?
And it seems to be that microPledge has set up a nice system where the incentives will lead to good stuff being produced in a fair way, with everyone benefiting. I hope the project succeeds. Our own experience
with donations suggests it may be a hard uphill climb, but a fulfilling and interesting one.
It will be interesting to see how the thing about leaving a project early or not continuing work on a project works out.. i can see that it might be difficult to say when a program is "done", and i can see differences in opinion coming apart regarding how to decide when someone should be able to receive the money pledged vs. ensuring someone doesn't stop working on a project while there are still deadly bugs in it. Of course that dilemna is found in all such contract programming projects and is not in any way unique to your system (in fact you may be able to work our a more democratic way to mediate such decisions and thus avoid the worst case scenarios where one rogue employer insists the program is never finished").
I hope you'll drop in and tell give us an update every once in a while