I very much doubt that any one entity will ever have 50% of the computational power. The botnet operators will bow to the whims of the community because it's the community that ultimately gives bitcoins value. What good is a giant load of bitcoins if you don't have anyone willing to give you something in exchange for them?
Eventually the largest merchants and money exchangers will control what is "standard" bitcoin.
Take the "50-coiners" scenario, and imagine that they manage to get 75% of the CPU power on their side.
But imagine that the biggest merchants and money exchangers are more conservative, and are in the 25% minority. I think they will be-- I don't think they'll be the ones in the business of generating coins (they'll be busy selling products or doing the exchange thing).
Well, the block chain splits. Transactions using coins minted before the split will get added to both block chains, and accepted by everybody.
Transactions involving "50-coins" (generated after the split) will be accepted on the 50-coin chain, rejected on the 25-coin chain. And vice-versa.
"50-coiners" would quickly find out that they couldn't get rid of their newly minted money because who wants bitcoins that are rejected by the biggest money exchangers or merchants?
If the big merchants and money exchangers disagreed, I bet you'd see Bitcoin clients that ONLY accepted pre-split coins and did no coin generation (since those transactions would be accepted by everybody). If it was never resolved, I think the number of Bitcoins at the time of the split would become "the number of Bitcoins, period," because most people will not want to use money that is accepted some places and not others.