From what I'm seeing, the senior plan doesn't include data. A data plan is required for the iPhone, which would be an additional monthly charge, if they'll let you do it. I would suggest going to an AT&T store and talking to a sales rep.
The price for the iphone is not dependant on which voice plan you get. As long as the voice plan is for either a new line or an existing line that is upgrade eligible, you can get the iphone 4s at the discounted price. As pointed out, the Senior Plan is for voice service only. If you get an iphone, you must also have a data plan.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.