iPhone 3GS activation error on iOS 5 upgrade - "No SIM card in phone"
here is the thing most of the answers work for those who have shsh blobs saved before but the thing is what if one has accidentally upgraded his/her iphone to the newer version and unable to activate it. 
in my country it doesn't work but he easiest way to activate your phone even without sim card is to call an emergency number like 911 or 112 and before you hear any voice press the power button and then on the notification bar area click again on the phone icon and hangup and you will see your home screen and well the rest is the rest.for video help search something like " Activating iphone with out sim card" on youtube.
hope i contribue something and sadly i couldn't get mine to work.