I know this is long dead but I see a few incorrect postings near the bottom that might confuse people about Shield Mastery.
Improved Two-Weapon Fighting (Combat)
You are skilled at fighting with two weapons.
Prerequisites: Dex 17, Two-Weapon Fighting, base attack bonus +6.
Benefit: In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a –5 penalty.
Normal: Without this feat, you can only get a single extra attack with an off-hand weapon.
That is a Penalty on ITWF, it is not the same as the base iterative attack bonus. Since it is a penalty it is ignored via Shield Mastery. ALL penalties are ignored for the offhand shield bash.
You still have 1/2 Str bonus and 1/2 PA bonus, so it is not like this is THAT strong to be honest... Even if you get cheezie and then use a LARGE shield, with power attack, and are fighting defensively, you get a bunch of bonuses with no drawbacks on your offhand shield attacks, but you still are doing below average damage compared to the 2hander.
To be honest Shield Mastery just makes it so sword and board can keep up with the pack, but it will never surpass a proper str based 2hander build. Personally I think the BAB prereq should be 6 not 11! Why punish Sword and board to 5 additional levels of mediocrity IMO.