Simple Q&A D&D 3.5 (by RAW) XXI

[KillianHawkeye]

The Improved Unarmed Strike feat is apparently enough as well for some things:

http://www.d20srd.org/srd/feats.htm#...dUnarmedStrike
Improved Unarmed Strike [General]
Benefit
You are considered to be armed even when unarmed —that is, you do not provoke attacks of opportunity from armed opponents when you attack them while unarmed.

Which might explain the theory that all things "considered to be armed" don't provoke attacks of opportunity for things that would if they were unarmed.

The phrasing of Improved Grapple, however, implies that even Improved Unarmed Strike isn't enough on its own (since IUS is a prerequisite for it).

Improved Unarmed Strike only does that for regular attacks (and tripping). Grappling, sunder, disarm, etc., still provoke without additional measures.