But beware that with some (or maybe even all?) firmwares R&D mode may cause faster battery drain since it may enable some additional debugging stuff or prevent device from deep sleep or something. Again there is not enough information but sometimes someone from @nokia.com vaguely suggests that enabling R&D mode may have some unwanted side effects.

having said that I had R&D mode enabled on my 770 for at least half of its time with me (since end of 2005) and did not notice any difference except easier poweron, green texts about system version and 'sudo gainroot' working.