The developers at slush or deepbit could change the limit today. It'd be risky to try to propagate a block today over the limit.
No, they can't. The rules of Bitcoin are not based on voting, only the ordering of transactions is. The only way they could do that is by becoming the effective developers of the Bitcoin software that all the users and merchants, etc use. As someone who mines a bit myself I _fully_ welcome them to try though! In fact, I'll gladly write the patch for any big miner that wants it.