The primary reason the Bengals decided to go with no fullbacks this year was Fui Vakapuna's shoulder injury that he suffered early in training camp. A shoulder injury for a fullback isn't a good thing, as the anti-Marvin would say.
The injury prevented Vakapuna from playing in the preseason, forcing the Bengals to use undrafted college free agent Joe Tronzo, who played well enough to sign onto the team's practice squad, for most of the team's fullback rolls before fully implementing Reggie Kelly and Daniel Coats against the Buffalo Bills.
Since he didn't play and since the team is already holding onto one injured running back taking a roster spot that's not playing, the Bengals elected to waive/injured the fullback.
As expected, Vakapuna cleared waivers. Now the fullback will go on Injured Reserve and miss the 2010 season, pushing back any NFL debut as soon as 2011.