When Google unveiled its instant messaging client yesterday, there was only one problem: Google Talk requires a Gmail account, which has been invitation-only since its beta debut in 2004. This changed on Thursday, however, as Google opened Gmail to anyone in the United States with a mobile phone.

The mobile phone requirement was designed to prevent Gmail accounts from being created by robots and stop spammers from signing up multiple times. Google will send an invitation code via SMS, which can then be used to register with Gmail. Only one account may be created per phone number. At the moment, Gmail only works with U.S. phone numbers, but Google plans to extend support internationally in the near future. Gmail, which offers 2.5 gigabytes of e-mail storage space, is already available in 29 languages.

Full story: BetaNews | Google Gmail (SMS Sign Up - US Only)