I'm surprised that they emphasize the fact that the keyboard can be invisible or small. Personally I think that's the most confusing feature because it leaves you tapping on the screen, or website as in the video. That aspect to me is not very intuitive as to whether or not I'm typing currently or trying to navigate the website by tapping a link, etc. Also, if it's invisible, how do you know when the keyboard is brought up on screen? Does it appear and then quickly fade away, or does it appear at all? Both cases seem unnatural at least to me, since I'm just not used to it.
Other than that, I think it looks pretty awesome. The multi-finger gesture typing looks pretty quick and intuitive.
I think that if it can be done right, that'd be the coolest feature of it.
I imagine it'd be possible to detect that you've got all of your fingers resting on the screen in the 'keyboard' configuration, and only 'activate' the keyboard when that's the case.
Like how with an iPhone it can distinguish when you press and slide your finger to scroll from when you press your finger to tap a link.
I think it'd be a cool feature because it'd give you so much more screen space for visible content (like when you're editing a document), and less of the context-switch that happens when an on-screen keyboard pops up.
In theory, there are lots of options for skins - fully visible/opaque, completely invisible (with some sort of icon to let you know it's active), any range of transparent, animated effects like heat haze, water ripple, etc. Kind of fun to think about, actually.
Watching the video it seems that the keyboard is active as long as a text-entry widget is focused, so it works exactly like a visible virtual keyboard - although introducing a mode with no visual feedback may sometimes induce errors.
Other than that, I think it looks pretty awesome. The multi-finger gesture typing looks pretty quick and intuitive.