Ok, this is something I knew worked in 3.0, wasn't clear in 3.5, and still isn't clear in Pathfinder.
I want to give some stone giants the PCs are facing Improved Grapple, but they need Improved Unarmed Strike. The problem is, it's a useless and redundant feat, because they already have slam attacks, which is whacking an enemy with their bare-hands anyway. So could a creature with a slam attack acquire feats with Improved Unarmed Strike as a prerequisite and bypass that prerequisite freely?
by RAW, yes they need improved unarmed strike. In our sunday game the Summoners eidolon had to get it for improved grapple.
However You can always make use of it and take other feats that need it like stunning fist, or some of the new ones from the APG assuming they are the sort of giants who could have those sorts of things.
Yeah my serpent shaman is not happy about that pre-req, but by RAW you have to have the feat
There is a difference between an Unarmed Strike and a Slam attack (although the difference is slight), making the mechanics different, warranting the Feat for Grappling.
An Unarmed Strike is a technique learned. A skill on how to attack and impact your target. It is a closed fist rabbit punch to the solar plexus.
A Slam attack is taking your massive hand and slapping and slamming it into your target, as a giant would do. It is "Hulk smash!"
The mechanics of the game basically say that you can't learn to grapple, at an expert level (avoiding the AoO) without learning how to focus your natural unarmed attacks into precise strikes.
You can get around this by giving the Giants the Dodge Feat and maybe a level of fighter (to increase AC and HP to counter the AoO they will get with out Improved Grapple), or just give them a level of Monk, so they get Improved Unarmed Strike.