Looks like Verizon won't allow iPhone users to have the $15 data plan. They'll be locked into a deal with the unlimited $30 data plan. However, in the future, it seems like they'll discontinue the unlimited plan and introduce a tiered plan similar to AT&T's.
I just looked today and the $15/150mb option is no longer available, only the $29.99 plan is available so that is a huge bummer and a deal breaker for me.
Most likely AT&T and Verizon will sort of collude to match the fees for data. They both want to charge as much as they can get away with for the iPhone.