I think this should just work but want to verify. I have an iPhone 3GS. My wife has some type of slider phone from AT&T that we bought about a year ago. She dropped it and broke the screen. I'm thinking of upgrading my line to the iPhone 4 and just giving her the 3GS. I can't test this right now since I'm working and can pickup an iPhone 4 before i head home tonight. Can i just take her SIM and pop it into the 3GS and have a working phone for her? Also, i think i pay $20 a month for unlimited Internet service on her phone, will all of that work on the 3GS? Or will i need to make account adjustments?