I think it might be more complicated then it looks on the surface because then it would take only 4.8 hours on average for a big pool such as deep-bit to plant invalid transactions and hack system with 3 confirmations, 11 hours to hack 4 confirmations, 26 to hack 5 and 60 hours to hack MtGox with 6 confirmations.
That's probably why it's bad if any single pool has close to 50% of the total hashing power. Deepbit's the only pool I know that has managed to strung 6 or more blocks in a row during the time I was recording down who found the blocks. In just the 1500 or so blocks, they had 4x 6 blocks run and get this: an 8 blocks run.
That said, it would still be hard for them to execute such a scam because there's no way for them to predict/guarantee they will get the next X blocks in any run. If they tried it repeatedly, I think it would be quite noticeable due to an unusual number of reorgs happening.
That said 4 and 5 blocks runs were quite common so Deepbit could possibly take a realistic gamble on hacking 2 or 3 confirmations.
I think it's more profitable to stay honest and rake in all those fees than try to spoof the block chain. Especially if it works and they get caught.Bitcoin will be declared worthless by the world.