How many consumer computers would it take to launch a 51% attack What is the hash rate of a standard laptop using the default client and what is it for the whole of Bitcoin? I am interested in this, because I want to understand what it would take to get to 51% of hashing power (is this what it is called?)
A standard laptop CPU provides around 2-8 Mhash/sec. An ATI 5870 video card, which has one the best efficiency/cost ratios, mines at about 400 Mhash/sec, and at the moment, the Bitcoin network has a total hashing power of about 12,460,000 Mhash/sec, which equals 31,150 5870 video cards.

So someone would need to invest about 10 million USD just to buy video cards, without including the cost of assembly and other hardware like mother boards, power supplies etc. It is possible to design custom hardware to do Bitcoin mining using an ASIC, which might have ten times better efficiency/cost ratio.

But it will be always too expensive to execute a 51% attack to justify just by the profit from the security breach because the Bitcoin network hash rate gets adjusted to the value of a Bitcoin -- in another words, people buy more mining hardware when the price of a Bitcoin goes up.