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.
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.