Someone visits area51 and wants to see what sites have graduated the staging stage and have been launched already...is this feature available?
Yes. There is a tab specifically for this.
This portion of the answer is before the launched tab was added. The methods are still valid, but not as important with the new launched tab.
A poor side effect of losing our domain names.
A51 will show you a graduated site, but only if you search for it, which doesn't help this scenario.
http://stackexchange.com/sites gives no indication as to the status of the site other than the icon not being the graph paper behind letters.
The current best way to do this will be to look at the footer here, on the trilogy, or on a graduated site as the footer only lists graduated sites.