The best incentive we currently have is the username claiming process. If/when project succeeds unclaimed usernames will be available to backers in the order that people backed.

What other incentives would you suggest? Happy to take suggestions. Today is the "announcement" phase, we are going to be collaborating w/interested folks and fleshing things out considerably more over the next month.

(p.s. I have been awake for far too many hours, so I am not sure I can effectively keep replying to everyone at the moment)

T-shirts and common schwag excite more people than one might think. Ask designer(s) to to donate something beautiful.

Kickstarter's limited rewards serve this purpose: discounted first year subscriptions, tote bags, stickers - the usual. It's a common strategy because initial momentum is so crucial for crowdfunding efforts.

Good luck, and get some sleep!

You want to attract developers. XYZ number of free API calls for your app is simplest and effective way to attract developers, if I understood your proposal.