I guess I came across this steering wheel vibration problem before, with my previous Honda... I traced mine to a sticky Honda brake caliper which was ever so slightly causing the brake to bind and rotors to warped. Well, I have just been chatting to a friend who has an imported Celica and he was telling me he had the same problem every so often. We decided to take a look and remove the front calipers and replace the rotors. The dust shields on the pistons were not split at all but one piston was completely solid. One quick strip and polish and coppa slip later, no more vibration.