MMA Weekly reports the numbers and exercises considerable restraint with their headline that ratings were "down slightly":
...the broadcast drew an average audience of 1.9 million viewers, according to Spike TV officials on Tuesday. That represents a significant decline in viewership since the promotion's most recent tape-delayed broadcast from England on Spike TV.
UFC 105, featuring Randy Couture vs. Brandon Vera in Manchester, drew an average of 2.9 million viewers to Spike TV. Prior to that, UFC 95, featuring Diego Sanchez vs. Joe Stevenson in London, averaged 2.4 million viewers.
Clearly Michael Bisping vs Yoshihiro Akiyama is lacking the star power of Randy Couture, but I would have expected it to perform comparably to Diego Sanchez vs Joe Stevenson. One factor may be the time of year of the events. UFC 95 was held in February, a much less competitive season for sports in the U.S. UFC 120 had to go up directly against college football, never an easy task.
All the same, it's not a good number and reinforces the theory that the UFC has over saturated Spike TV.