I had that same problem on my early 02 WRX around the 30K mark. They replaced the faulty plug, boot, and coil pack under warranty. I can't say how long it would have worked as the car was totalled at 37637 miles.
I'm not sure if is actually a problem though. It may be that the ECU is too sensitive and trips the CEL when it shouldn't
I think some dealers have sent the ECU out for reprogramming to fix it permanently.