This might be an equally effective but less centralized approach. What do you think?
How does one detect a fork? Some guy in his basement can create a fork with his own new code but be useless due to most transactions going with the main fork, but it would still be a fork.
Run several versions of Bitcoin and if they start to mutually deviate over three blocks then that is either a fork or is, a least, sufficient reason for merchants to go into safe mode. Sounds to me like a business opportunity.
Actually, if this is implemented, it would make it much safer for alternative full Bitcoin clients to operate because we could detect the inevitable chain splits quickly and safely.