Love the tipjoy concept, but development in this direction seems to be 'slicing a thin pie even more finely'.

Also, there are issues of 'motivation crowding' to consider -- most sites/projects that rely on user contributions could hurt themselves if they made it seem that financial rewards were the main reason to contribute. Cf: Google Answers vs. Yahoo Answers, among others.

The real challenge, I suspect, is creating a cultural expectation that certain kinds of online content/service should be supported by casual tipping. That's a matter to be addressed with marketing/trendmaking/usability/ubiquity/patience, rather than complexifying the process with backend payout-sharing.