Watching Match of the Day 2 tonite, it appears to me that both Rooney and Henry were off side when scoring. But both goals were taken from back passes.

Does the off side rule not apply if it is a back pass?

Not offside from a back pass otherwise defenders could just keep passing it back at will.

If it's played by the opposition (defence) back then no you cannot be offisde. The ball has to be played from one of the attackers team, if it takes a defelection or a defender gets a foot to it, it is still offside.