tacoma speedometer will set mileage on a cluster for you for $80 plus shipping. Great customer service there. They goofed due to a discrepancy between 05 and 06 sti clusters that they had never encountered and they fixed it for free, and offered to pay overnight shipping both ways. Very friendly/helpful on the phone and all that jazz.
Also there is a guy around here that says he's done it to a WRX cluster himself though as of yet I do not believe he has made this info available.