USSD is indeed implemented now in PR1.2. "Classic" service codes seemingly are not.
Don't confuse it with SS codes (see comment #47 and various comments by Lassi). SS codes is not officially enabled in PR1.2, mainly because most useful actions are more conveniently done by UI in (Settings->Phone settings) such as call forwarding, Call ID enable/disable etc.). These are legacy codes, so don't get bogged down by this -- just visit your "Phone Settings". Less useful ones are less useful, anyways. If you are *still* interested to play with SS codes "dialing" via Dialer (despite the User-Interface), there is an ester egg available to enable it in PR1.2. You have to work it out yourself or tip Lassi with some beers before he reveals it :).