I believe this is what you are looking for:

http://eee.asus.com/en/transformer-infinity/features

I've never used one but a lot of people on HN seem to swear by them.

Except it's more expensive, and the screen and CPU are not as good.

I know, I'm waiting for the refresh. All told it's $200 extra counting the keyboard dock but I consider that totally worth it, especially since there's an extra battery pack in the keyboard.