I have a 1999 Olds Alero 3400 (3.4L) Automatic with 161,000 miles that idles at 500-600 rpm regardless whether the A/C is on or off. Sometimes it will idle at 750 rpm and smooth out, not that the lower idle is that rough. The DTC P0506 is the only trouble code that comes up which references to the IAC Valve. I have an emission test that the car needs to pass here in Texas but with that check engine light on it will not pass.
P.S. Sometimes, at cold start up(50-80 degrees) it will lope slighlty and die. On restart, it will generally smooth out at the low idle condition.