Google Voice is now open to anyone that wants to signup. No more invites required..
source: Google Voice Blog
A little over a year ago, we released an early preview of Google Voice, our web-based platform for managing your communications. We introduced one number to ring all your phones, voicemail that works like email, free calls and text messages to the U.S. and Canada, low-priced international calls and more—the only catch was you had to request and receive an invite to try it out. Today, after lots of testing and tweaking, we’re excited to open up Google Voice to the public, no invitation required.
Over the past year, we’ve introduced a mobile web app, an integrated voicemail player in Gmail, the ability to use Google Voice with your existing number and more. Over a million of you are now actively using Google Voice, and many of the features released over the past year (like SMS to email and our Chrome extension) came as a result of your suggestions, so thanks!
source: Google Voice Blog