No attack of opportunity is granted.
According to the Pathfinder's RSD attacks of opportunity are triggered only by:
- moving out of a square in the threatened area of a hostile creature. Exceptions are: the 5-ft. step, the withdraw action, and teleportation (but not necessarily casting a teleport spell).
- performing a distracting act while in a square in the threatened area of a hostile creature. This usually include:
- attacking with a ranged weapon,
- attacking unarmed,
- delivering a coup de grace,
- performing a combat maneuver,
- casting a spell,
- using a touch spell (if the spell is harmful you are considered armed; you don't draw AoO from the target, but sill draw from other enemies)
- activating a spell-like ability,
- activating a spell-completion magic item,
- drinking a potion or applying an oil,
- loading a crossbow,
- moving heavy objects around,
- picking up objects,
- sheathing a weapon,
- retrieving a stored item,
- standing up from prone,
- escaping from a net,
- activating or activating spell,
- locking or unlocking a weapon to a gauntlet,
- preparing to throw a splash weapon,
- using many skills.
Appearing into a square because of a summoning spell seems to not fall in these two categories.
However, appearing in the middle of an fight may or may not be considered a distracting act, according to how your group interpret Summoning effects.