http://www.businessweek.com/technology/content/aug2005/tc20050824_9823_tc119.htm

But starting today, invites are no longer needed to get a Gmail account. In a separate announcement, Google says it will take the wraps off the service for users in the U.S. Gmail will be open and free to U.S. users but will require that they use a mobile phone capable of receiving text messages to prove they’re legitimate users. When they sign up, Harik says, Google will send their phone a text message. While it may not be fail-safe, Harik says the measure is an effort to minimize Gmail being used by spammers and for other types of abuse.