The published spec conceives of halving the number of bitcoins a block miner receives every n years (4 or so?). Right now, a block miner will get 50 BTC. In a year or two, they'll only get 25. A few years later, 12.5. The Sum of the BTC generated before the block generation gets really really little is roughly 21 million.

As has been pointed out copiously today, that's a convention that BTC clients and users all claim to agree to. It seems likely to me that they will continue to agree to it, and it also seems likely to me that the major hurdle will be the first '25 BTC/block' downgrade. If the community survives that, the others will likely follow easily.