It is crystal clear that tripping with a special trip weapon or with Improved Trip does NOT provoke an attack of opportunity. Likewise, it is clear that a normal character tripping with an unarmed attack DOES provoke an AOO.
There is a grey area, however, with regard to a character tripping without Improved Trip, but with either Improved Unarmed Strike or a natural weapon.
(Or even, I suppose, an un-discharged melee touch attack spell prepared.) The rules for Trip say "This provokes an attack of opportunity from your target as normal for unarmed attacks." Since such characters do *not* provoke an AOO for their normal unarmed attacks, it can be argued that they would not provoke for the trip attempt, either.
It is up to the DM whether this logic sways them or not -- just be sure and be consistant whichever way you go.