Let's do some fun math. I have E6750 CPU with 5425 BogoMips on each core. I do 2200 khash/sec so it's 2200000/(5425*2) ~= 203 hash/Mip. Now look at the difficulty; it's 712.88486455 now. So the network does 712.88486455*2^32 ~= 3061817179056 hashes per 10 minutes. Now we can estimate its power in Mips, 3061817179056/203 ~= 15082843247 Mips ~= 15 Pips. Here is a world most powerful supercomputer: http://www.top500.org/system/performance/10184
It has only 2 Pips in peak and maximum 1.7 Pips (don't really get the difference between max and peak, maybe max is an average maximum and peak is a one time performance shot). I don't remember what's the threat of the overpowering of the network but it looks like the attacker will need about 9 such computers for successful overpowering. I guess it's highly ineffective for such tasks and want to congratulate us all for participating in such a big and powerful network (although we do pretty absurd job in mathematical sense).
How about 20 Radeon 5970s and OpenCL client? 40?~$700
* 20 == $14,000
Maybe $400 for rest of computer parts * 20 = $8,000
Only $22,000 + cost of electricity + closed OpenCL client to completely pwn the network.
ArtForz? Do it!!! Hurry, before an open source client becomes available!
Although, it is unlikely he would do it because it would probably cause the value of Bitcoin to fall to $0.00000001.
Instead he is pursuing most profitable method while preserving value of Bitcoins. ^_^