Not sure what model iPhone you have, but I have a similar problem with my old 3G (jailbroken, full-size SIM). The short story is that I also have an iPhone 4 and was playing around with inserting the micro-SIM from that phone into the 3G using an adapter. As other folks have touched upon, some adapters are not the best fit and there can be a slight ridge (a few mils) at the interface between the inner edge of the plastic adapter and the outer edge of the SIM. This can make it difficult to remove, sometimes requires some "fiddling".
Well, the bottom line is that my adapter broke inside the SIM slot, probably due to excessive fiddling. I had to open up the phone and remove the residual plastic, and noticed that one of the SIM contacts was "distorted". Also keep in mind that while all SIM's are electrically identical in their I/O configuration, the physical patterns of the contacts are close but not indentical.
You can guess where this is going- my AT&T SIM is recognized by the device, but none of my other SIM's (T-mobile, H2O, Red Pocket) are recognized, whereas they had been working just fine in the past. The one SIM contact point in the socket is distorted just enough to (apparently) just miss making contact on 3 of the 4 SIM's I have.
Have you opened the phone to see if this is the case? If you can't do it, there are plenty of free-lance iPhone techies out there who will at least look at it for $20 or so, or if you're adventurous the Internet has plenty of tutorials and tool sets available for you to go DIY.
I was also told that it is in all likelihood not repairable, or more accurately not worth repairing in my case (new motherboard required).
I am El Supremo, master of all I survey, a legend in my own mind...