I just spoke with Verizon Wireless. It turns out that their "no obligation, month-to-month" data plan included with the iPad/MiFi bundle is significantly different from a "pre-paid" plan and significantly different from AT&T's iPad 3G plan.
With AT&T, you choose -- each month -- whether to buy a month's worth of iPad data. Think of it as a "pre-paid" data plan that is (1) discounted for the iPad and (2) limited to the iPad.
Think of Verizon's plan, however, as buying a contract-free MiFi 2200 and getting a $139 discount for buying an iPad at the same time. After subtracting the price of the iPad, the MiFi is $129 instead of its $269. For comparison, you can buy the MiFi by itself for $99 with a 2-year contract (and no iPad).
Unlike AT&T, Verizon bills you each month until you CANCEL your data plan. Because you're not under contract, there is no termination fee, but you'll have to pay a fresh $35 ACTIVATION FEE to restart your plan.
AT&T advertises their iPad plan as a month-to-month decision. If you're going on a trip, for instance, you can buy data for that month and then not buy data again until your next trip. It would be unwise to do that with Verizon's plan, since it will cost you the same amount to re-activate ($35) as it does for a month's worth of data -- not to mention the hassle of re-cancelling.
The only benefit, then, to choosing a Verizon iPad + MiFi bundle over an AT&T iPad 3G is that you get access to Verizon's larger 3G coverage map and a MiFi 2200 that you can use with any WiFi-enabled devices (up to 5 at a time). I confirmed that you can resell the iPad and continue to use the MiFi. The MiFi does not require the iPad in any way.
But if you're thinking of buying the bundle and selling off the iPad just to get the discounted MiFi, consider this: Buying a 2-year contract with a standalone MiFi discounts the MiFi $170. However, the Early Termination Fee for that 2-year contract is only $175! The 2-year contract is only a $5 risk! In other words, if you end up keeping the data plan active for two years, buying the iPad bundle makes the MiFi $30 more expensive than buying the MiFi by itself!