If you want complete freedom to use your mobile service and not want to bind to a single service provider then sim free unlocked iPhone deals are the right solution. First popular sim free unlocked deals are started with '16 GB 3GS white iphone' but now sim free unlock deals are also available with iPhone 4 which allows you to use your own SIM card and continue using your existing cell number.
With sim free iPhone you can use other card that behaves like a sim but you can use it as per your convenience and can be removed at any time.
The other advantages of using sim free unlocked iphone are you can use it anywhere globally with desired network connectivity, convenient unlock facility and it is positioned such that the usual sim card and the sim connectors do not get affected and cannot affect this card, you are free to detach the card and use it whenever you like and as it is compatible with every card you can attach any sim card along with the unlock sim card.