No SIM Card installed error in iPhone 3g
Consider the possibility for your SIM card to have failed. It happened to me and I had to go to a phone store and get a duplication of it. Here in Europe it costed me 6€ to have a duplicated SIM. It took about an "hour" to be activated, but after that, it started working. 

Try using your SIM card on another phone if you have one around or go to an Apple Store and see if you can test the SIM there.