As you probably know, the block reward is set to halve somewhere around late 2012 - early 2013. It's impossible to say with precision when this will happen because block generation time is somewhat unpredictable, but the first block to feature a 25BTC reward has the number
209999.
We should expect that the mining market will be very mature by that time, the profit margins very thin and the price fairly stable. Since miners can't control the price on the market the halving of the block reward and thus of the mining revenue will be seen as a negative effect by the miners, many will no longer be able to cover their expenses. A period of cutthroat competition should follow, where the last man standing can hope to return to profitability, but not before the difficulty also drops by half. This is necessary since where you previously needed one mined block to cover your expenses, you now require two blocks, so a single block should twice as easy to found as before. In other words, half of the hardware will be tossed before the bloodshed is completed.
But it doesn't have to be this way. Since the miners are more or less unanimous that the change hurts their bottom line, they might chose to dissent from the planned schedule and keep the reward at 50BTC. If they simply patch their clients to remove the halving, it has little effect; as soon as the first fake 50 BTC is claimed the blockchain will split:
- the original blockchain will continue with a 25 BTC reward, as implemented by all clients, exchanges, merchants and other parties who have no incentive to patch the reward schedule; this chain will continue to be protected by the fraction of the miners that chose not to dissent and play by the rules
- the split chain of the mutineers with the fixed 50 BTC reward; since only the mutineers accept the patch, the coins mined here are not tradable on any exchange and are effectively worthless.
The revolution has failed and the miners are forced to return to the legitimate chain and cannibalize each other as planned by Sathoshi-san
This is where the plot gets thicker. Assuming a high fraction of miners decide to accept the patch, they can attempt to force everybody else to accept the change: they can fork the blockchain
and attack the legitimate chain at the same time. The attack will consist of extending the original blockchain past block 209999 with empty blocks. The mutineers will hardcode their miners to refuse any reorganization and hammer away at extending the legitimate chain with empty blocks only. As we know, if they have 50%+1 of the computing power in the legitimate chain, this will prevent any transaction from clearing in the original chain. Meanwhile a small fraction of mutineers will mine the alternate blockchain to keep it somewhat secure.
I would expect such a move to be widely advertised beforehand and alternate client builds to be made available so that users can "upgrade" to the fixed reward algorithm. If someone refuses or forgets to upgrade he will find that his client is no longer able to send or receive any transactions. Once most users and services have switched to the alternate chain, the attack on the original chain can decrease in intensity. The minority honest miners will eventually stop defending the original chain since they are unable to get any block they mine to stick in the chain - the majority evil miners refuse to extend their blocks.
What I like about this attack is that miners are not risking too much by attacking the original chain: if the revolution fails they have not wasted resources by mining empty blocks. They are anonymous and it's very hard to prove that mining for an empty block on purpose is a security attack from a legal p.o.v - it's certainly allowed by the protocol. There's also a strong incentive for miners to conspire to change the rules, the individual benefit is in line with the collective benefit, unlike the normal mining game. So if the mutineers can amass a majority of the hashing power of the network, there's little doubt they can force a rule-change.
Ok, so what stops miners from doing this right now, and increase the block reward to 5000 ? Nothing really. I just believe the block 209999 shock to be a strong catalyst, maybe enough to make the majority of miners participate, and change the rules only slightly so as to maintain overall confidence in the system.