This limit is set by miners and the reference client (and the various clients that copy the reference client's limit). I'm not sure of the exact minimums neces

This limit is set by miners and the reference client (and the various clients that copy the reference client's limit). I'm not sure of the exact minimums necessary to change the limit, but roughly speaking if you could convince more than half of active bitcoin users and enough miners to control more than half of the network hash rate, you could change the limit. However even this wouldn't necessarily change the limit in the way you expect. It would create a hard fork where you would have two versions of bitcoin. All coins made before the fork would exist in both versions, but coins made after would only exist in their respective version. So you'd have A-bitcoins with a limit of say, 1 billion, and B-bitcoins with the old limit of 21 million. Whether anyone would still use B-bitcoins depends on how many clients/miners decide to stay on the B-bitcoin branch.