I came across this note from the Engineering team at Facebook which talks about how they managed to smoothly launch their recent 'pick a username' feature. The title of the note is, appropriately enough, 'Hammering usernames' -- this is of course because they were expecting their infrastructure to be hammered.
In the note I saw for the first time a name for a strategy that teams I've been involved with have applied before: 'dark launching'. Essentially, dark launching is releasing a new feature to a subset of your users, mostly with no UI changes, but otherwise exercising all the parts of your infrastructure involved in serving that feature. A good strategy to apply when you're dealing with massive, large-scale deployments, and when you want to see how your infrastructure behaves in conditions that are as close to production as possible. Because remember, there's nothing like production! Your careful load/stress testing exercises in a lab environment ain't gonna cut it.
The note from Facebook has all sorts of other nuggets of wisdom related to massive infrastructure deployments. I recommend subscribing to the RSS feed for 'Engineering @ Facebook's Notes'.
While googling for 'dark launching', I also came across this very good post by Dare Obasanjo. Recommended reading.