My sons 05 WRX starting idling at 2500 rpm when hot this spring. Per Valhakar's (Dave's) recommendation, quoted below, I adjusted the cruise control cable to put more slack in it. Initial indications on the first day are that the problem is fixed. My thanks and kudos to Dave for identifying the problem for me. It is great to have this site and be able to quickly find the solution to a problem.
Hats off and thanks to Dave.
It is the Cruise control cable. It came from the factory too tight on many of the 03-04s. When looking at the throttle body from the front, it is the cable that loops to the left and attached to a small black box on the wheel well next to the turbo.
This is the issue. As the turbo heats up the area the box/cable deforms and tightens. Loosen the cable to have at least 1/2 of slack while cold and the issue will go away. Do NOT loosen the cable on the right as that is the actual throttle cable.