http://www.d20pfsrd.com/feats/combat-feats/shield-master-combat---final
Prerequisites: Improved Shield Bash, Shield Proficiency, Shield Slam, Two-Weapon Fighting, base attack bonus +11.
Benefit: You do not suffer any penalties on attack rolls made with a shield while you are wielding another weapon. Add your shield’s enhancement bonus to attack and damage rolls made with the shield as if it were a weapon enhancement bonus.
So far I am under the vague impression that this means, as long as one is wielding, or using, a weapon in one hand, their shield bash would ignore attack penalties from sources such as two-weapon fighting, power attack, Combat expertise, and other class abilities which impose such negatives.
I need a council ruling on if this should stay as is or should be converted to apply to only a select few sources that impose an attack penalty.