Vessenes is correct in practice, and somewhat correct in theory. SPV clients, which many expect the majority of Bitcoin users will use, do no validation of transactions, leaving the protocol rules up to miners to decide by majority vote.

In practice even users of full clients can be forced to follow whatever protocol rules miners decide upon, because without a majority of miners the blockchain isn't secure.

Now would the people with mining hardware make a protocol change that the economic majority were opposed too? If they did, what would happen? We don't really know - that's a political, sociological and economic question, not a technical question.