Very close if not already there. If AMD or Nvidia would take some power saving methods or if AMD were to pull off with Mullins and Beema with their APU then we are acutally not too far off.
I do not see much OEMs outside of Razar actually market such a device though. So there maybe pushback from some OEMs for coming to a trade off conclusion that can be pushed to market.
15W MAXIMUM CPU/GPU specified envelope (as I played with my Surface Pro devices) puts heat dissipation at very reasonable levels with the fan being quite quiet in comparison to the game sounds or even using headsets. It can be lower of course, but this would diminish the possible performance envelope from medium to high computing at a cost for catering to very low areas of the envelope.
Graphical performance is had with around 30 FPS at 720p or 1080p given various low to medium settings tinkering and most importantly, how textures are utilized.
The only other issue, is human interfaces and input. PC gaming always has been designed around a sliding pointer device and a keyboard - and to an extent controllers and joysticks. Much like Windows 8's interface transition to entail all possible inputs while keeping cohesion and software compatibility and interworkings, past games cannot utilize a pure touch screen - thus a Tablet Gaming PC would still need to offer keyboard or pointer device options (either a pen, keyboard built in or easily attached and same with a mouse). For sure, it is just a simple port and marketing of current touch screen games to touchscreen capable PCs, laptops, and Tablet PCs. Steam can already tag games with such labels.
New, original games, would be also made for the Tablet Gaming PC, but much like the console controller, the dedicated commonly used gaming inputs would be on screen and obscuring the display and again limited in input keys compared to a keyboard. There would be some novelties in swiping and multi touch input (I definitely would like to see a spiritual Elite Beat Agents on the PC, God knows when Nintendo would get around to it).
But, there is an example out there if executed for the Tablet PC realm, would take off. Things like the Wii U, has shown such a form. If the Wacom Cintiq Companion has set aside bezel space for the physical keys and circle ring, there is no reason for an OEM to not make it - it is just up to the OEM to fine test their sourced components, tweaking their drivers to work with the soruced hardware, and possibly account for cooling and weight balance - much like Microsoft's Surface Team went into designing the internal layout of the device.
In short, it is all there. OEMs need to actually put the effort to integrating hardware and use the right software to allow new and old games alike to work. And if a gaming Tablet PC can be made, then for sure, a light weight gaming ultrabook can be had (at current console performances~ish).
Edit:
Video of current already in market capability
https://www.youtube.com/watch?v=gX4BSRriVQo