I just did a casual check of the site's user list, and noticed that a good number of Area51 committers (self-identified as expert or research level) have not yet registered with the beta site.
In addition, I peeked at some avid user's SE-network profile activities, and noticed that their activities come in bursts: 10+ days of silence, then 10+ good quality questions and answers on a few single days. It seems they are having a busy real life doing important businesses.
Combining those thoughts (or perhaps just my own subjective interpretation), I'm a little bit uneasy about going into public beta automatically - based on a pre-programmed countdown clock.
Can we set a public beta threshold based on the number of committers actually registering on the site and have at least one activity (commenting/voting included) before going into public beta?