My guess was that the provocation of the RC-135 was not the goal. Most likely they were hopeing that a SK or a US fighter would get in the mix and there would be some shooting. This could be spun into an act of war and moved attention away from Irag. Makes you wonder why NK is increasing their activity and rhetoric the closer we get to invading Iraq.....
You'd have to ask Spook86 for sure, but I'd assume that they were in and out before any U.S. or ROK fighters could have made it on the scene. A 150 mile dash to a slow moving transport plane and back could probably happen before any friendly fighters could be scrambled and sent to intercept.
They wouldn't want to wait for any fighters anyway. Hitting the RC-135 would be an easy kill, and they could fly home and claim they were provoked by an intrusion into their airspace. A U.S. or ROK fighter attack would probably leave the North Koreans sans four Migs.