I think the part of this that got me pondering is that typically a basic prototype or initial version would be open sourced, people would play with it, and then funding or sponsorship would come along. Projects like Redis, WordPress, TravisCI, jQuery, Node.js, Ruby, Phusion Passenger, and PHP come to mind.
Your approach is different in asking for money up front and then promising to open source only when the project is finished.
While it seems you are awesome guys, much loved, and do good work, it's that leap from the previously established model that gets me wondering if it's a good approach. Maybe it is and I just don't realize it yet. Maybe I'm being nostalgic. But if this project were already open sourced and you were asking for funding to make a serious go of it, it would, to me, seem an easier sell than "we want to build X, give us the $ we need, and we'll open source it when we're done."
I suspect I'm just being a bit of an old fogey, but without questions come no answers.. ;-)