This query should be redirected at Microsoft, as Mr. Alan has indicated NDAs prevent him from divulging such information.
As I read the progression of these posts, I speculated, as well as others, that this was indeed the case. However, for those who tend to read between the lines, the important piece here is what Mr. Alan did not say.
With all due respect to Mr. Alan, my opinion is that if the internal rendering was @ 1280 X 720, he would be able and willing to state this. Would an NDA prevent him from supporting Microsoft's ethos of a 720P minimum HD resolution? I think not. However, if the internal resolution was below this "so-called standard", he could not comment, as this would have all sorts of implications as to Microsoft's HD requirements.
What's interesting here, is that I think we've all learned that Microsoft's 720P minimum requirement is an OUTPUT standard, NOT a rendering requirement.
***720P DISPLAY RESOLUTION***
I've looked closely at Microsoft's rhetoric regarding HD, and I have not once read anything about an internal rendering requirement; only HD requirements, which can now be read as "HD display requirements".