The only devices that you'll be able to try it out with are a couple USB modems for laptops...mainly focusing on Verizon's business customers.
You won't be able to buy a 4G iPad or a real iPhone 4G for at least 6 more months...
Verizon previously stated that consumer 4G devices (i.e. smartphones, tablets) would be available before "mid" 2011. These new devices are expected to start showing up, starting with the Consumer Electronics Show in Vegas next month.
I would suspect that a 4G MiFi will come long before any iPad or white iPhone is equipt to run on Verizon's 4G network. Which would essentially be a 4G iPad...this capability exists with a Sprint Overdrive MiFi Hotspot.