I'd also point out that this very idea has been tried several times already, never with much success.
I worked on a project called Fundry, that was exactly this. Developers could add projects and setup paypal etc attached to it, then users could "put their money where their mouth is" so to speak and directly fund features they wanted. The idea being features would pay for themselves and developers wouldn't end up implementing features nobody wanted.
But alas it ended up failing, as the poster above mentioned, we weren't the first to try it, kickstarter beat us to the bunch by about 2 weeks but we continued on hoping we were targetting a different market than kickstarter.
We ended up open sourcing the code in case someone else wanted to run with the idea, I wonder if the new mob used it at all? We'll probably never know.