How strong is the bitcoin network?
Three months is about the useful life of an ASIC miner. The cost of all practical mining (not including wasteful mining done by botnets and the like) tends to equal the value of all practical mining. So an approximate rule of thumb for the cost of an attack is the value of all Bitcoins mined in three months. Since the block reward is currently 25 BTC, Bitcoins are around $700, and a block is mined around every 10 minutes, you're looking at roughly $200 million. Using existing equipment is impractical -- you need ASICs.