A WebmasterWorld thread has a webmaster who noticed his site was spammed and he is anticipating his site to be penalized by Google, and the other search engines.
He asked if he should start early and submit a reconsideration request with Google. A reconsideration request would notify Google of the hack and spam and let them know about it as soon as possible.
So far, all the feedback is that he should not wait until he is actually penalized to submit the reconsideration request. Yes, I know - he was not penalized yet but the belief is that he will be. And if he is not, what is the worst that can happen? This is assuming he does not have real spam on his site.
So he has to clean up the hack and the spam and then submit a reconsideration request.
Forum discussion at WebmasterWorld.
