adding an existing iphone to my plan - any reason not to?

[mattkime]

i have an iphone 4 that is under contract until february. my girlfriend has an iphone 3gs that is no longer under contract. it seems that we'd save a fair amount of money by putting her on my account. Any reason not to? (would it limit our upgrade options? etc)

How does it work cost-wise? Its kind of difficult to find this out on att's website.