DRM is not the mainstay of the HDMI connection. HDCP (High Defination Copy Protection) is. HDCP has been in the HD spec since day one. It has changed, evolved, changed some more, been delayed and so on, but it has always been there.
It is a complicated implemetation which basically states that everything in the chain of the HD data path must provide HDCP question and answer tags.
This is a big over simplification of what happens, but basically a DVD player would have to ask the TV if is supports HDCP (and the corresponding HD tag). If the TV does not give back the right answer, then the DVD player must only supply whatever low res image the DVD provider decided on. In some case, the DVD disc maker may have chosen to give you a blank screen through the HDMI port. In other case, you may only get a low resolution image.
Currently, that tag which controls the resolution is not being used. The committee decided to delay its implementation for a year. Once it goes into effect on the media, then the current players and TV will have to honor the flag. Of course, they currently do not.
How that situation will be dealt with is still in committee. If the tag was implemented today, no one would be able to watch a high resolution movie on a DVD (HD or BluRay) which expected that tag.
It gets even more complicated when you want to watch an HD movie on your computer. Currently, your DVD ROM drive, your video card, and monitor must support HDCP in order to play an HD content DVD in high resolution mode, once all the resolution tags are enabled in HDCP.
Fun stuff, eh?