It doesn't exist. You MIGHT be able to create something of a Hackintosh tablet, but you'd probably end up losing a significant amount of functionality, and it would be a violation of the Mac OS X EULA.

I'm just not so sure that you'll see anything like that out of Apple any time soon. I also expect that the Windows 8 tablets will be a huge letdown. Everyone is likely banking on the idea that embedded hardware will improve dramatically by the time Windows 8 is ready to go, and there will likely be some kind of stripped down interface for tablets. My guess is that they will be incredibly slow and cumbersome when they do come out, or the tablet version won't be a "full" Windows OS.

This is exactly why Apple won't be releasing one any time soon. Maybe once embedded hardware has improved a little, they may think about it. I could easily see them eventually merging the iPad and MacBook Air, which have a lot of market overlap, but probably not for at least another 2-3 years, maybe more.

Apple is all about the "experience". They aren't like Microsoft who is concerned more with exposure and widespread use. Microsoft will let people slap Windows onto just about anything so long as they make a few bucks off the licensing fees. Apple controls both the hardware and the software, and at least so long as His Steveness is calling the shots, there's no way they'd ever put a full Mac OS X client on a tablet. That ever important experience would not be up to his rather high standards. At least not with current technology. In a few years, when someone comes up with a better mobile chip, maybe. Almost everything else they need is in place. The RAM chips can be soldered directly to the logic board, and the SSD used in the new MacBook Air's is about the size of your average DIMM, so it's plenty thin. They just need something a little better than the current Cortex A9 CPU.