Tried to second your post on HTC's support forum but their forum software is misconfigured and my email server is rejecting their mail because it is not a fully-qualified hostname. Meh. Like you, I don't hold out enough hope of a fix to bother with the registration process.
You mentioned flip to silence, but there is actually a setting on the Evo Shift 4G to silence the ringtone when the phone is moved. It's under settings - sounds, and it is "quiet ring on pickup". Since the action of flipping the phone involves moving it, I wonder if you are seeing that setting working when you flip your phone over. My phone stops ringing (if it's not in a pocket) when I pick it up off the table. It doesn't decline the call, it just stops the ringer.
Edit: eh, I made an allowance on my mail server so HTC's misconfigured SMTP server could send my activation message and let me register.
By the way, I posted a workaround on Android Central that works for the way I use the phone. I created a silent ringtone. Then I set the phone to vibrate as well. So in my pocket I feel/hear the vibration but I don't get the ridiculously-loud rings. But I can hear my notifications at normal volume. When the phone is on my desk, the vibration and screen lighting up are enough of an alert for me. Obviously this won't work if you ever keep the phone in a bag or somewhere that you can't hear or feel the vibration, but it works for me as a pocket user.