So we have rolled out our first client site in Mercury. Everything went exceedingly smoothly and that was awesome. The only exception was that I could not get mail() to work on my new vps . The error messages were crap, as in there weren't any, and ultimately (and after many many hours) I resorted to using PEAR::Mail. That still throws warnings but the mail gets delivered.
Next up: user-to-user messaging and adding friends.