Is there any way to ask to be notified when a site goes live without actually comitting to participate in it? I need this because (a) I might not be up to participating in all the sites I'd like to, and (b) you can commit to a maximum of three sites.
Once a site goes to the commitment stage, that becomes the default tab. The definition tag is still available, however. (See http://area51.stackexchange.com/proposals/1369?phase=definition for instance.) That tab has a separate "Follow" button.
When a new site enters public beta, the "followers" of the proposal will receive an email notification.
You can follow as many sites as you wish and you would be notified about each one, once they leave private beta.