Posted 05 July 2009 - 13:54
Yes, you should remove it. There's nothing on it that's required for the iPhone to function, and it can be traced back to you. Anyone's that purchasing an iPhone second-hand will have their own SIM for use.
In addition, make sure you use the "Settings > General > Reset > Erase All Content and Settings" feature to remove any personally-identifiable information stored on the phone.