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.