When, after a shutdown, booting with the HMDI cable attached, the Astro first vibrates briefly, then the LED next to Q comes on in blue for a few seconds. After that, HDMI kicks in, sometimes showing some binary garbage, followed by a generic Android logo and after that, the SIM PIN screen, the lock screen and, finally, the home screen, all in glorious 720p (which is very cramped and looks weird on a 50-ish inch TV...)
When I, after a shutdown, held volume down and turned the Astro on, with the HMDI cable attached, I never got any HDMI out.
It is quite possible I did something wrong. As I recall, I slid off of the volume down button a few seconds after the Astro powered on, so maybe I should try again and hold volume down pressed for longer? Is volume down + power on even the correct sequence to get to the recovery mode on an Astro? I basically just noticed I didn't get any HDMI on the first attempt, forced a reboot and e-mailed Planet.