What's Google up to with Gmail SMS signup?

It's apparently to prevent spammers from opening fraudulent Gmail accounts (like it was so hard to get an invite or fifty anyway), but the perpetually beta webmail client now has an SMS account authentication system available for use without an invite—just punch in your cellphone number and hit it back with the code it texts you. Of course, actually being able to use Gmail on your cellphone might be a more practical development, but now that a Gmail account can be used for other slightly less interesting things than plain old email, we can't help but wondering whether there isn't something bigger going on behind all this than just getting everyone on a Google version of Passport.