I've looked around a bunch but none of them have HDMI input. I'd bet that there won't be any either. HDMI input is not something people want for a tablet with poor resolution. Most folks only want to display the tablet on a large screen like a TV.
Also, most chipsets for tablets only offer HDMI output and most tablets are barely using the reference designs for these chipsets. So for a company to spend money and design extra circuitry for HDMI inputs when there is practically no demand for such, is probably not going to happen.
It won't exist for quite a time. Best option today is to make 2in1 construction, so main screen will be serviced by normal TV chip (MStar, etc). And tablet will draw all output into overlay. May be something like special shared memory :-) But tech is not ready yet. All tablet processors are just toys now (Tegra 2 included).
But it is very interesting to look inside Ninja and similar products.