Normally, a temporary ID is set, but this would have enabled you and your friends to SMS back and forth without worry. You would have been assigned a number in the 406 area code once you sent your first SMS message. However, Google found a bug, and had to pull back the feature. According to their blog post:
Oh snap. Last night, we started rolling out a new feature to Gmail Labs that lets you send SMS text messages right from Gmail. It combines the best parts of IM and texting: you chat from your computer and reach your friends no matter where they are. Your friends who are away from their computers get your messages as texts and can peck out replies on their little keyboards. It was pretty cool for a few minutes last night when we were sitting around texting each other.Two weeks? Not too long to wait is it? Or is it, in today's frenetically-paced society? Oh, and for those who don't know: CRBT means "Crying Real Big Tears."
Then we found a glitch. When you'd try to turn it on, it wouldn't fully enable. We thought about keeping it out there -- bugs and all -- but the experience wasn't that great. So, in the spirit of Labs, we've pulled SMS chat back to fix it, and we'll get it back out to you as soon as it's ready -- probably within 2 weeks, so stay tuned.