There has been some concern about the fact that transaction fees (in their current form) are not an adequate replacement for the current 50BTC block reward. Gavin has said as much, as have others. The issues of incentive misalignment are described in other threads.
I'm curious: as the 50BTC->25BTC block reward reduction approaches, there is a strong incentive for the miners to modify their clients to simply ignore this rule, continue producing 50BTC-reward-blocks, and ignore any blocks which try to reduce the reward. It requires changing only a single line of code!
It is in the best interest of every miner to do this, and the miners decide what winds up in the block chain. Therefore, I suspect that it might actually happen. Is there reason to believe otherwise?
Obviously if more than half the miners agree, they can change any aspect of the bitcoin protocol. But this is a bit different: it's easier to get a bunch of people to agree to not make a scheduled change in the protocol than to make a new change. It's also easier to do this if omitting the scheduled change is in all the miners' best interest. So I think that eliminating the reward reduction is a lot more likely than any other protocol modification. "Oops, I forgot to reduce the reward I pay myself!" It's also less likely to produce panicked "OMG they changed the rules of the game" and a crash in the BTC exchange rate, since what's really happening here is less change, not more.
Additional note: although pooled mining makes it easier for this to happen, no "agreement between pools" or collusion is actually needed. Think of it this way: the block that is supposed to reduce the reward to 25BTC is very likely to be a forking point in the blockchain. Every miner will decide for him/herself which side of the fork to work on. The fact that there is more reward on the non-reduced side of the fork means that a rational miner will choose that side. The fact that more miners work on that fork means it will ultimately become the "real" blockchain.