I have iPad 2 3G and the same thing happened to me. Did everything that can be done to take that no sim message. I read in one discussion that it might be the cover of the iPad. Then it hit me. I bought a new cover days before that no sim message, and realised that it was the only change I did before the no sim message was shown even if there is a sim. So I took off the cover for 3 days then inserted the sim again. It worked! Now I have sim access again!