My estimate:

• Lets say 40% of the people are going to do a naive estimate of 25 based on half of the average of random guesses between 1-100
• Lets say 20% recurse one level and guess 12.5
• Lets say 15% think they're clever and guess super low like 0/.5/1
• Lets say 15% of people are going to be anchored to 40 and guess 20.
• And lets say the remaining 10% just guess more or less randomly, the average being 50

This results in an 'average' guess of ~20.5, half of which is 10.25. I actually guessed 10.5 as I used slightly different estimates.

Of course, had I known this was posted on LW, I'd have used higher probabilities of low guesses.