I have thought of one way the block chain could be defended in an attack...

I could start up a "Casascius Block Registry" by myself, which would be some RPC web service where I sign all the blocks that I think are good, and allow people to query me for my signature for any given block.  No one has to care, nor does anyone have to pay attention to what I'm doing, and more than likely, I get very few hits from only a few curious geeks.

A few other individuals who think alike might do the same thing.  So after all, you have a bunch of obscure geeks signing blocks, nobody cares about these signatures, nobody queries them.  Consider it a form of digital masturbation.  Maybe a dozen people do this.

Then along comes an attacker.  Panic ensues, along with widespread disagreement as to what to do next.

One option on the countermeasure table becomes to incorporate querying the "Casascius Block Registry" into clients, as well as the other dozen registries created the same way, as an assistive tool to decide which blocks are legitimate and which should be discarded.  This effectively puts the whole of Bitcoin into the hands of a dozen individuals, which of course is far from decentralized as Bitcoin would like to be, but would be better than the status quo in the event of an attack that experiences success.

By starting my registry long in advance, I would have already defined an RPC query protocol and established in people's minds that such a database exists as a countermeasure.  I'll have a sense of established legitimacy and reputation for having Bitcoin's best interest in mind, and by analyzing what I sign, people will have already had a sense for how I decide whether a block is good or bad before they point their clients at me as a source of validation for blocks.

If it successfully mitigates a 51% attack, others start to realize what a good idea it was, and start their own block registries so defensive power isn't in control of a dozen individuals for very long.  And if the attacker gives up and the registries aren't needed anymore, the world can go back to simply using the longest proof of work as is done now.