iPhone 3GS activation error on iOS 5 upgrade - "No SIM card in phone" I upgraded my iPhone 3GS to iOS 5 a few days ago. The upgrade went smoothly; however, I'm having trouble activating my phone. 

My wireless provider is O2 in the UK and my O2 SIM is in the phone, but when I connect to iTunes I get this error: "There is no SIM card in the iPhone you are attempting to activate. Please disconnect and insert a SIM card in the phone."

I've also tried activating over WiFi, but am getting an error message like "Activation servers are temporarily unavailable".

iTunes on my Windows x64 laptop is latest (10.5.0.142) and I've tried rebooting both my laptop and my iPhone several times.

Any advice would be highly appreciated.
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.