Character screen displays + Damage as being applied after +% Damage, but it is only applied after damage has been applied to the target. Consequently it does not increase damage but is still taken into account when calculating life returned by % Life Stolen per hit and Life Tap, and mana returned by % Mana Stolen per hit: this means life and mana can be returned even when attacking an Immune to Physical target (provided Life Tap is cast or the target has >0% drain effectiveness). 
Not what I was expecting either :P
Further clarification from Onderduiker:
Bash + Damage never gets applied to its physical damage; it adds nothing, whether your target has Damage Resist 0% or is Immune to Physical. It only gets applied when leech is calculated, after physical damage has been applied.
The application order when calculating physical damage is as follows:
Base damage (ethereal bonus)
Weapon damage (weapon's +% Enhanced Damage, followed by + to Min/Maximum Damage)
Total damage (off-weapon +% ED e.g. Bash and its Stun synergy)
Final damage (Critical or Deadly Strike)
Resistance (DR %) Damage has now been applied to target
Bash + Damage
Leech (drain effectiveness and difficulty penalty are multiplicative)
Based on the (lying) character screen, Bash + Damage is meant to be added after off-weapon +% ED... but it's not. Even if it were, at that stage in the process it wouldn't make a significant contribution to damage unless you were bare-handed or equipping a low damage weapon.