Since Tiger made the switch to the Nike ball prior to the 2000 U.S. Open at Pebble Beach the major victories per ball manufacturer are as follows:

Titleist - 14
Nike - 12
Callaway - 4
Strata - 1

From the 2005 Masters through the 2007 PGA (noting that 2005 marked the end of the 'Tiger Slump') the breakdown is as follows:

Nike - 5
Titleist - 4
Callaway - 3

Times are most certainly changing. A few players each year move away from Titleist and have for the last six or seven years. That said, with continued improvements by TaylorMade, Bridgestone, Callaway, Srixon, and Nike we'll likely see Titleist's "#1 ball in golf" title go away in a few years.