Been having the same problem trying to find an answer too. What I have found is that ASUS aren't an approved MHL supplier (or words to that affect) although I think that the I/O developer Nexus 7's have an HDMI port and
MHL active in the USB? Some people saying it has been turned off in software so that Google can sell their Nexus Q (possibly meaning it will be enabled if people make enough noise) and others saying it isn't included in the hardware (why include something that costs money when you're trying to cut costs to a minimum). Although in my experience if you have something in prototype/first edition boards it is often cheaper to keep it and disable it in software than remove it and I can't imagine MHL hardware being that expensive.
The problem is that this is all speculative and no one actually seems to know! I have an MHL cable for my Samsung S2 so would be keen to use that if possible.
That being said the Raspberry Pi with Android Transporter is a much neater solution and doesn't mean leaving the tablet sitting on the floor where it risks being stepped on.
I guess we'll just have to wait and see what comes in the post