Here's my opinion:
The IAC is very finicky and even then, can just stop working for no reason. If there are pockets of air, or low coolant, it will start to hassle you.
Did you top the coolant back off when you were done, and burp the coolant? I mean REALLY burp it.
Like.. really, really burp it.
Other than that, make sure the 3 sensor connectors are plugged in correctly (TPS, IAC, Idle switch((name?)).
Also, a really bad vacuum leak will make it rev high and die. It would have to be pretty obvious though.