It depends on who you ordered it from, Apple or AT&T?
AT&T will send you specific activation steps and you would activate the new iPhone 4S using the activation information you receive from AT&T.
Once you complete the activation step, just swap Micro-SIMs and you are good to go for now. The phone number moves with the Micro-SIM. Your wife's number will be tied to the new Micro-SIM and you can reuse your existing Micro-SIM from your current iPhone 4.
Apple will have a different activation process so check back on Friday for the latest details. Last year the iPhone 4 was tied to the phone number ordered againt and had to be activated against that line before you could swap the Micro-SIM. In iTunes you were presented with the phone number that was not an edit field and you were prompted to enter the last 4 digits of the account owner's SSN.
Note: Either activation step listed above will result in your wife's current SIM being deactivated so make sure she is aware of what's up when you try to activate.
Dave