So, unless you have 51% of the computing power, you can't decide which fork is the valid one.
That's a feature, not a bug.
No, I'm saying that even with 51+% of the computing power, you CAN'T decide which fork is the valid one if the wizards don't agree.
Take for example the current situation.
"If you are using Bitcoin-Qt/bitcoind version 0.7.2 or earlier, you must take action before 15 May, 2013. If you do nothing, you are likely to be left behind and will be out of sync with the rest of the Bitcoin network."
Notice it doesn't say that if 51% of the computing power decides not to upgrade... never mind.
It says, we've decided. Come along or not.
So in other words the client with the largest install base ultimately makes the rules. The fact that source code is available is utterly irrelevant to the vast majority of users as they would not have the expertise to derive anything from it. So rather than having monetary policy dictated by the Fed, Bitcoin users may ultimately have it dictated by those responsible for the dominant client application.