Powerful Build grants the Goliath the ability to wield weapons that are one size larger than their form. It doesn't effect natural weapons.
I don't know if that is from some eratta somewhere, but nowhere in the Races of Stone does it say that. The only thing it indicates as not being affected are his space and reach. It also states that it stacks with any powers abilities or spells that change the creatures size. Now I didn't specifically look into the wildshape bit so take that for what it is worth.
My interpretation of the Powerful Build trait is that the character counts as a size larger in all instances where it would be beneficial to be bigger. That means they get a grapple bonus, carrying capacity is doubled, they can wield larger weapons, and their unarmed attacks do more damage.
My interpretation of the Powerful Build trait is that the character counts as a size larger in all instances where it would be beneficial to be bigger. That means they get a grapple bonus, carrying capacity is doubled, they can wield larger weapons, and their unarmed attacks do more damage.
Does the powerful build racial trait allow a character to take advantage of feats for size Large or larger creatures, such as Awesome Blow? Would a half-dragon goliath qualify as Large for determining whether it has wings?
No and no. The powerful build racial trait (found in the goliath entry in Races of Stone and the half-giant entry in Expanded Psionics Handbook) spells out exactly when the character is treated as one size larger than normal:
1. Whenever subjected to a size modifier or special size modifier for an opposed check. This includes grapple checks, bull rush attempts, and trip attempts.
2. When determining whether a creature’s special attacks based on size can affect him. This includes improved grab and swallow whole.
3. When determining what size of weapons the character can use.
That’s it. The character isn’t treated as one size larger when determining space, reach, Hide check penalty, or for the purpose of qualifying for anything that requires size.
Some feats, prestige classes, or other game elements may specifically allow a Medium character with powerful build to qualify as if he were Large. Such examples are specific and intentional exceptions to the normal rule.
Does the powerful build racial trait change the damage the character deals with unarmed strikes and natural weapons?
No. The powerful build racial trait allows the character to “use weapons designed for a creature one size larger without penalty” but doesn’t say anything about changing the damage dealt by his unarmed strikes or natural weapons.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.