You need to call. Its annoying, and a total drag. Online you cannot switch off the iPhone plan. AT&T reps confirmed when I called them. Each time I switch from my 3GS to my Android G1 I need to call AT&T and have them switch the data package. Thus, I decided I will not be sim swapping very often. Also it makes the billing looks screwy on your bill because they charge you for X iPhone days and X other data days and blah blah blah.
I *wish* the AT&T system would smarten up, and detect what phone you are using and add the package for it since they cost the same.
Ugh.