I have an unlocked iPhone 4 and an international sim card. If I am at a wifi hotspot or on the airplane with free wifi will I still be charged for cellular use or do I remove the sim card and the wifi will still work?
You won't be charged for data but will get charged for SMS or if you make or answer voice calls. If you turn on Airplane Mode in Settings, then re-enable WiFi, voice and SMS will be disabled. You don't have to remove (and possibly lose) the SIM card.