We know botnets can get really big if they're run by clever people. The question I have is, if someone creates a clever botnet and turns it on once mining has become unattractive financially to those who pay their own electric bills, isn't getting over 50% pretty feasible?
If a botnet guy would get over 50% network power (which I found unfeasible since machines with decent GPU performance are usually not in the hands of computer illiterate people), they would already make so much money off it that they wouldn't even need to cheat. 1000 btc roughly every 2 weeks, that's between 10k - 20k USD depending on how the price will move after mtgox reopens.
Lots of Dells and Apples and HP's used by average Joe's have Radeon 5xxx and 6xxx cards in them, but I think you missed my point. I'm thinking of 6 months from now when, if BTC doesn't appreciate significantly, mining won't pay for the electricity required to do it. At that point, if the computational growth from legitimate miners slows or goes negative, and mining becomes mostly a botnet business, those same folks will have the power to turn BTC upside down at will. I'm not saying that they're want to or that they could profit from it, but it's a real possibility.