Tomsula has been the defensive line coach of the 49ers since February of 2007. He was San Francisco's interim head coach for the final game of the 2010 season. He also was the head coach for the Rhein Fire of NFL Europe in 2006, but this is his first gig as a full-time NFL head coach.
Tomsula replaces Jim Harbaugh, who left to become the head coach at Michigan University following four successful seasons in San Francisco on the strength of a dominant defense. Tomsula helped that cause while coaching the likes of Justin Smith, who was a quality player with the Bengals, but has played at a Pro Bowl level under Tomsula.
This also takes away one more head-coaching position that could have been filled by Bengals offensive coordinator Hue Jackson.