Yes, definitely. Take a literal reading: it says that, while you have combat advantage against an enemy, you get a +2 power bonus to damage rolls against it. This means all of the damage rolls. If you take this stance and additionally somehow have the Ranger daily to make five attacks, all five of them will get the +2 to damage.
Note that it doesn't count towards damage that didn't involve dice. You don't have to have actually rolled them - you get it for critical hits, for example, even without a magic weapon or High Crit - but dice must have been involved.
EDIT: In general, you can read pretty much any rule, power, ability, feat, or whatnot in 4e as literally as possible and get it right. The handful of real controversies tend to stem from places where things are not well-defined (or perceived as such).