I don't know about the 2008, but my 2006 wagon did the idle thing too...
Figured out what it was. When I turned on the car, I was immediately (within seconds) getting it in gear and moving along. I noticed that when I let the car sit for 7 seconds prior to getting on the gas or putting it in gear, it behaved normally.
Along with the idle, the car would lunge forward at times, when I hadn't even adjusted the gas pedal. It would also lag on occassion too.
When I turned on the car, buckled myself in, and waited for the RPM to reach normal idle, and then got going - all of these issues went away.
If I had to guess, I'd say it was the ECU being retarded because I gave it input prior to it setting itself up for operation. It's hard to explain what I'm saying - I know some of this sounds bizzarre. Basically, I drove the vehicle prior to it initializing... if that makes sense.
My 1996 does it too, but to a lesser degree. Same work around takes care of the issue.