Pinch the vacuum hoses to see if one brings the idle speed down. You can also find a mechanic with a scanner to view live data, and look at the "IAC steps". The Engine Computer controls idle speed, in part, by controlling how much air bypasses the throttle blade. It's a controlled vacuum leak. The computer can set the automatic idle speed motor to one of 256 positions. The higher the "step", the more air it lets in.
For a properly-running engine, step 32 is typical for desired idle speed. With one cylinder misfiring, you'll usually find it at around step 50. If you find yours at step "0", it means the computer sees idle speed is too high and is trying to bring it down, but without success. That points to a vacuum leak. If you find it well above step 32 to about step 35, the computer is commanding the higher speed, and you have to determine why. Even though intake air and coolant temperature sensors are extremely simple and reliable, an incorrect reading from one of them could make the computer think a higher idle speed is needed.
It's hard to imagine why replacing spark plugs would cause a difficult-to-locate vacuum leak, but if there is one, a smoke machine is real useful in finding it. That allows you to inject a white, non-toxic smoke at 2 psi into a vacuum port, then you watch for where it sneaks out.
At any time during this service did you disconnect the battery or allow it to run dead? Is the Check Engine light on? Have you checked for diagnostic fault codes?
Monday, November 10th, 2014 AT 1:04 AM