The new Flattr service could be integrated with StackOverflow.
Like someone's question / answer / comment? With Flattr integrated, you could reward them with actual cash.
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This isn't going to happen.
Joel and Jeff talked about this on many, many occasions on their pod casts. They confronted Jason Calacanis (big believer in micro-payments) in podcast #56 where they hashed it all out. The conclusion was that Stack Overflow uses an "expert economy" (reputation, prestige, education, etc) and even attempting a "token economy" would essentially destroy Stack Overflow.
I think this type of feature was suggested in the past and met with rejection (my gut tells me this will be evident in the coming votes). Sounds a bit too much like Mahalo Answers.