GMail's viral marketing campaign has been very well executed. It was a simple technique. Start with a small user base (many of which were Google employees), then repeatedly give out small numbers of invites to the existing GMail users for friends and colleagues. The pool of users grows exponentially, but in a controllable way, and every user feels like their few invites are scarce and precious.
I has assumed that they were restricting supply of the accounts because it was a beta test, but there's more here. The artificial shortage of supply creating such demand and buzz that people have been paying to get invites. People who have a GMail account feel exclusive, elite, like part of a club, all because of the shortage of supply. Now everyone wants a GMail account.