That all does sound inline with my knowledge as well but I think the conclusion in the debate as i remember it ended something like "its more profitable to mine the bitcoin blocks than it is to attempt a 51% attack as most retailers, casinos, and other big businesses won't allow you to run off with something until all 6 confirmations are confirmed which would be about the worth if not more than the hiest and if it isn't then that business is just ignorant and stupid for not waiting for all 6+ confirmations.
For large transactions, the client could update the "confirmed" code. At least 6 blocks and ((average fee + mint reward) * number of blocks) > (value of transaction * some constant).
The cost of 1 block's worth of POW can be estimated as the average block payout. This ensures that the cost to brute force a reversal (constant) times more than the transaction.