This API update talked about the HDMI out feature and how to code to it. I got the impression that applications had to be coded to specifically use the HDMI out, and that by default, the media would also try to display on the phone's screen. Iirc, the doc suggested shrinking the on-screen media to 1x1 pixels and shoving it in a corner, allowing the screen to be used for informational displays and/or large touch controls.
I was concerned that HDMI out would only work through the cradle. I have a Zune HD, which also outputs 720p through HDMI, but only through a $50 cradle. I knew that going into the purchase, but I didn't realize the cradle would only work with external power, and it had a hard-wired antenna for HD radio reception. Not portable at all! I'm hoping the EVO will be the solution I've been looking for to output 720p via HDMI cable directly from the phone. I have lots of 720p home movies to share with my family! But what's the port? Mini? Micro? I've never heard of micro-HDMI. I can't find confirmation anywhere.