Bewenched - 8:05 pm on Mar 26, 2010 (gmt 0)
I recently had to to this myself.
We moved our servers and I accidentally blocked some of googlebot (the ones that run stealth got a 403). I hadnt seen a penalty yet because of it. I went ahead and put in a reconsideration request and told them what had happened. The day after putting in the request I saw that WMT was showing some of the 403 errors in their crawl stats.
Not sure if I averted a penalty, fearing that they might think we were cloaking, but we havent missed a beat in the rankings.