Based on time-zone adjusted (but still preliminary) fast nationals, Super Bowl XLIV averaged over 106 million (106.5 million) viewers and is set to overtake the 1983 series finale of M*A*S*H (105.97 million) to become the most-watched program of all time. Even more impressive as the MASH finale was only one hour and the Super Bowl averaged that many viewers for more than a 3 hour period.
Looks like the MASH finale was 2.5 hours (I don't remember it being that long, but I'm old!) according to one of the commenters, and indeed, the population in the USA was much smaller then. Also it was the pre-People Meter days when Nielsen figured out viewer averages based on diaries.
FYI: per Nielsen 83.3 million TV homes in 1983 vs 114.9 million in 2010
With Super Bowls, it easily eclipsed the previous most-watched Super Bowl, last year's Cardinals vs. Steelers game that averaged 98.7 million viewers.