after activating iPhone 4, how to wipe 3GS?

[btownguy]

I'm all converted over to my iPhone 4. I plan on keeping my 3GS as insurance in case I damage my 4. How can I do a reset to factory wipe of my old 3GS without activating it now that my new phone is active for my phone number?

 

[gethyphyyo]

Click restore, when it's finished and asks you how you would like to set the phone up, unplug the sync cord.