Here's an idea: Reject blocks larger than 1 megabyte that do not include a total reward (subsidy+fees) of at least 50 BTC per megabyte.
As a protocol rule?
I find that worse than an automatic adjustment of the max block size. You can't set prices like that. You can't predict what 50BTC will represent in the future.
No, it does make some sense. With this type of system in place, the security of the network would be based on a BTC based reward. This way we would always have similar security level
relative to the actual value of the monetary base.
Bitcoin is bound to become less and less secure relative to the value of bitcoins, since the total reward for miners is going down. Right now it doesn't matter at all because our network, just like Mike Hearn said, is actually extremely secure relatively speaking. It will continue to be that way for a while yet, but not forever. At some point the security level might become questionable. If there was something in place in the lines of Gavin's suggestion, the security of the network is pretty much guaranteed.
I do agree with Mike again that it's a bit questionable to just set it at 50 BTC. We really don't know how much mining is "high security", we only know that what we have now is quite enough. It would be wasteful to pay more fees for more mining if we don't actually need it.