Still that wasn't the impression I got at all. What you be the metric for AAA vs AA vs A? You do understand that waiting is no protecting from a 51% attack right? The attack chain would be built in secret. It wouldn't be released until it is longer than the valid change. So a block rated quadruple AAAA would instantly be erased without warning by a longer chain and the transaction replaced.
The most important metric would be when the block was received. If I receive a block that tries to replace a block 6 or 10 or 100 blocks ago on the chain I already know about, and I have no reason to believe I've been segregated from the network at large, I'm not going to vouch for it.
By the way, I don't have to be the only person "signing" blocks. If pools signed their own blocks, the signatures in the blocks themselves could also be taken into consideration as self-vouching, as the attacker is not going to be able to fake those signatures.
With respect to the argument that the attack chain could/would be built in secret... this is 100% correct. If I were MtGox, I would probably program my bitcoind to simply shut down if it ever came across a need to reorganize 6 or more blocks, so that the desired reorganization results could be sorted out manually. (in effect, this could already be accomplished with just a few lines of code, and the desired outcome signaled via another block checkpoint, just like what the namecoin exchange did when faced with a threat).