I was wondering... if you make some kind of change to the windows phone registry (after jailbreaking the phone), will an hard reset bring back the reg to the original unmodified state?
Yes. The default registry is part of the ROM. Couple of weeks ago I added one innocent key to the theme registry key and it crashed my phone. Couldn't open any third party app, including the Registry Editor, so no way to change it back. Hard reset and several later I was back where I started. Won't be doing that again any time soon.