my wife just got an iphone 3GS. we both have apple IDs and use a macbook together, but her iTunes on her iPhone shows my username and the credit amount that I have. When buying a song or an app, my user account info pops up. How can we have her username and log in information appear on her iPhone?
Are you using the same user on your MacBook? If so, you're probably using the same iTunes too, which probably is set to your Apple ID. To use a different ID, you need to switch accounts every time you sync the iPhones. To sign out, choose "Store" > "Sign out" in the iTunes menu bar.
Thankfully, this whole hassle will be over when iOS 5 is released.
You could also simply authorize both accounts to run on your computers at home (store--> authorize this computer), and then, on the actual IPhone, go to settings--> store--> log out, then just log back in using her account.