If i want to buy an iphone...does it has to be unlocked for the Sim card to

work or it doesn't matter? I currently have a palm centro from AT&T and want to buy an iphone. Can i just buy an iphone and insert the sim and go or does it has to be unlocked in order for it to work?
Do I still need to add the data plan from at&t or not.
I was also thinking of buying the phone off of ebay.

If the iPhone is locked to a different network you need to unlock it. To unlock, I suggest you can approach Simpleunlocking.com and specify the country,locked network of iPhone.Based on the availability you can get the permanent unlock service along with free instructions.