Just a few note pertaining to repeat comments as a current VZ customer:
This is the same as verizon's current unlimited data plans for smartphones - it is not a special concession due to the iphone.
Verizon's smartphone data plan is indeed unlimited, you can go over 5GB without surcharges. It is their tethering and laptop 3G devices which are capped at 5GB, and they clearly state as much. However, if you are repeatedly going over 5GB with your smartphone, you are most likely using your phone as a tethered device via a jailbreak or third party app, and that can be seen as abuse of the service (in fact it is, it's in the TOS) and you may be contacted about it.
Tethering is not free, it costs an additional $15 a month with the normal smartphone plan. The tethering fee is generally the cost of a normal laptop device data plan minus the cost of your current smartphone data plan - the idea being that regardless of what device you use, everybody pays the same for laptop data access.
My experience has been that the cost comes out close to the same between the two for most people unless you get tricked into a weird plan. Do your research, and especially make sure to check for any employer discounts you may be eligible for. One benefit of my plan is that my employer discount for VZ plan add-ons is better than the base voice discount - that means since Verizon treats data as an add-on rather than having separate smartphone voice/data plans, and the base discount was already a little better, my overall discount with verizon is much steeper.