Invite codes are a really smart way to control growth. After a few weeks, anyone “in the know” enough to really want a Gmail account, can have one. At the same time, Google can control the service’s rate of growth. There’s no explosive rush, and if anyone notices something isn’t scaling as well as it should, they can throttle back the issue of new invitations until the problem is solved, without too many sleepless nights.
I met Chris Uhlik of Google. He's the program manager of Gmail.... I asked him why they only give out a few Gmail accounts at a time. Remember, I thought it was sheer marketing genius. Turns out it wasn't. Turns out they don't have enough server capacity to deal with everyone who wants an account. So, when they buy a new server and get it installed they hand out another round of invites.
(As an aside, who do you have to shag to get invited to FooCamp?)