For instance, lets suppose BFL sells just $5 million dollars worth of their "coffee warmers". (a highly conservative number that'd probably leave them bankrupt after they paid for the ASIC development costs) That's about 33 thousand coffee warmers, or 234TH/sec. Suppose the attacker decided to requisition a whole bunch of computers to attack Bitcoin, for instance by asking Amazon or Google "nicely" One 4-way Opteron CPU can do about 115MH/s, so for your 51% attack you'll need about 1 million CPU's. If you're renting from Amazon, that's costing you something like a million dollars an hour, assuming you could even get them to let you rent that much computing power. The capital cost of all that computing power is also in the range of hundreds of millions of dollars, heck, easily a billion dollars with server farm overhead.
Actually they would need more than 117GH/s, because once they add their 117, the total hash rate goes above 300, they still don't have 51%.
Doh! Yeah, in general that's not a safe assumption, because in theory miners drop out as profitability drops due to the new hash power, but I'm assuming a sudden attack. In that scenario you actually have to provide an equal amount of hash power to the existing network. So all my already conservative, BFL labs goes bankrupt because they don't sell enough hardware, estimates can be doubled.
As it is, the idea that miners will drop out as profitability drops is probably not all that true either now that mining profitability is mainly a function of capital costs rather than marginal electricity costs.