I am having the same issues with a 32gb iPhone 4 on 02 upgraded and activated yesterday it started happening last night so I just reseated the sim in the holder and it came back. Happened three times again this morning & a restart sorted it for a while but now I just get the no sim message all the time even if I restart or reseat the sim. As i'm at work I cant get to my computer to reset until tonight so very frustrating.
I have spoken to O2 and they claim my sim is ok & I have put it into a nokia using the Micro Sim adaptor supplied and I am getting full signal and no issues
There is another active discussion about this at
http://discussions.apple.com/thread.jspa?threadID=2476328&tstart=30