Here in Iowa those mobile speed traps usually have cameras in them. I personally got a ticket from one mounted on a freeway overpass on HWY30.
As for the license plate covers, they do not work. If the inspectors see a blured plate they look at the negative image. Take a photo of your plate covers and look at the negative, you will be amazed what you see...
Man, I wish I still lived in Texas. There you could pass law enforcemnt at 20mph+ over the limit and they would not flinch. Here in Iowa you get 5mph speeding tickets and the highest speed limit in the entire state is 65mph (limited access freeway only, most 55mph).
-nut