The idea behind this is so brilliant that it's a wonder it wasn't developed sooner: Jing Yang of eico Motion Lab in China has created the Smart Keyboard, what he bills as the world's thinnest keyboard for the iPhone.
It's based on the concept behind the iPad 2 Smart Covers. Roll the keyboard onto the screen when you want to use it, then roll it back off when you're done. Like the Smart Cover, it attaches to the bottom of an iPhone with magnets and can be used as a stand.
This hasn't been developed beyond a concept video yet, but check it out below to see the potential. This would be a fantastic Kickstarter project. You can follow any project updates via SmartKeyboard on Twitter.