The roll out of the iPad 2 brings additional monthly data plans - that is if you're willing to change cell phone providers. The follow chart summarizes the various plans available.
|Chart from All Things Digital|
The issue of monthly fees and long term contracts will become increasingly important and increasingly complicated as more mobile devices become available. I've mentioned before about the inevitability of "tiered pricing" - different rates for different types and quantities of data.
Purchase price will be less important as vendors attempt to lock users into long term, complicated, expensive contracts.