Cool, but how the hell does that work?
How the hell does what work? The backup of your 2FA keys? They're backed up to the cloud, simple. It's synced with some unique ID--probably your phone # or your e-mail address or both. But don't worry, you can't just take someone's 2FA just because you have their #. It sends the confirmation to your e-mail AND as an SMS that you need to verify. On top of that you need the password to decrypt the keys.
My rough understanding of how they do it is they store your encrypted keys on their servers. I'm not sure how big of a tinfoil hat you may wear, but to some this may be a big hole in security. If done properly they should not have any knowledge of the decryption key. It's described more in detail here:
http://blog.authy.com/backups
The way I see 2FA backups is more for convenience. You don't want to dig through your Titanium Backups of Google Authenticator or whatever.
With that said, there seems to be a button that says on on your Google 2FA settings page that you can move to another Android device. I'm not sure if that moves ALL your keys or only the Google 2FA token. Either way, I find Authy a lot easier to use, as you can set it up on your tablet or multiple devices if you even wanted to.
In the end you can totally turn backups off and multiple devices off and store everything on your phone and only your phone. I'd trade just a little security for a huge gain in convenience, especially with 2FA. Remember, even if someone has your 2FA token, they need your password