I think it is just the opposite. I think Steve Jobs has wanted a $15 data pricepoint since day 1, and AT&T could never deliver it, but between the negotiation for the iPad and with this, AT&T has likely extended their exclusivity deal, partly by presenting this lower price point, which means the iPhone is now available and affordable by more people.
With a $97 wal-mart price on the 3GS and $15 a month data, it is a dang attractive package to people who before felt it was too expensive.
a lot of people want a smart phone but don't want the $30 data plan. $15 for light usage like facebook is OK for them. my wife uses 100MB a month or so. a lot of months 50MB. i even told her i was going to cancel her 3G since she never uses it