When is the Bitcoin payout for solving a block going to change? When is the Bitcoin payout for solving a block going to change?  What happens when overnight it is half as profitable to process a block?
It halves from 50BTC to 25BTC block reward after 210,000 blocks, which at 6 blocks mined per hour by my calculations works out to be in 358 days, which is around Dec 13th, 2012.

It's likely that the number of active miners will reduce substantially at that point, but within a relatively short time the difficulty should adjust down so that approx 6 blocks per hour are mined again. In the time it takes this to happen, transactions may take somewhat longer to be processed and confirmed.