Amazon is reportedly in the midst of beta testing a VOD HD implementation for TiVo devices. The quality is apparently "outstanding," and the progressive downloads allegedly handle surround sound quite well.
We're keeping this news in rumour land for now, but there is visual evidence out there that this beta test is indeed happening. Our tipster David over at Zatz Not Funny vouches for the accuracy of his sources (who also supplied the pictures).
Other things of note: The current library is (unsurprisingly) pretty limited, but at the very least we do get our man Tony Stark here. We also don't know if this functionality is coming soon to other devices, like that plucky Roku box I've had my eye on, but it would also be unsurprising if this feature was scaled out. [Zatz Not Funny]