Found this on the ToyotaNation site several months ago, but don't remember the link. It worked on my 07 non-NAV Highlander, don't know whether it's valid on the 08 or any NAV equipped models. ALLCAPs entries are steps not usually included in directions provided in forums.
1. Turn the ignition on. Do not start the engine (leave your foot off the brake). Cycle odo/trip reset knob until display reads ODO only (no Trip A or B shown). Turn the ignition back off. Wait a couple of seconds.
2. MAKE SURE THE DRIVER'S SEATBELT IS NOT FASTENED.
3. Turn the ignition back on. Do not start the car (no ready light, leave your foot off the brake). As soon as you see everything light up, press and hold the odo/trip reset knob and do a long 12 second count.
4. Keep pushing the odo/reset knob in WHILE FASTENING THE DRIVER'S SEATBELT (awkward, but possible-I hooked the belt over the armrest beforehand). As soon as you fasten the seatbelt, the ODO display should change from XXXX miles to "Seat belt buzzer on". Take your finger off the odo/trip reset knob. Display should cycle to "Seat belt buzzer off". Turn the ignition off and unfasten driver's seat belt. That should do it...