I think I am going to sell my 3GS on Ebay now that I have had my iPhone 4 for a few weeks and it's going good. Would the phone be considered unlocked then since I replaced the 3gs with the 4? Also would I include the sim card or take it out?
No, The iPhone isn't unlocked because you're switching phones. Also, Don't sell your SIM along with your phone, it has your contract and service on it.