When I heard the first iPhone wasn't 3G, I immediately nixed it. With 3G, I breifly entertained changing to them (but both negative experience with ameritech/cingular - now AT&T - along with missing features I consider essential to my usage killed that thought quickly). So, I am not surprised that others chose to hold out for faster mobile broadband speeds.
Nor do I find landline usage dying out. There are many good alternatives out there. Infrequent users, road warriors, and those who don't want to be constantly annoyed by telemarketers may choose to use a cell phone instead. Cable broadband users (and those who can get DSL service without land line service) might choose a VoIP service - after all, I pay Packet8 $210 a year plus $4 a month for my unlimited national calling plan.
However, I do know a lot of US phone fees do go to subsidizing phone service to rural areas. Often, these people do not have adequate broadband service or cellular coverage, and thus they have no choice but to pay high fees for what most urbanites would consider sub-standard service.
Thus, the iPhone has little to do with a drop in land line usage. It is better cellular coverage in general along with other alternatives that are eating into these sales.