Surprisingly, there doesn't seem to be any catch to this. Simply apply. Although I'm not sure if this only applies to LTE devices.
Verizon double-data LTE promotion revealed - SlashGear
It should be no surprise to you at this point that the DROID RAZR is about to be one big gigantic deal for both Verizon and Motorola what with its slick nature and LTE connectivity, and here’s another reason to get excited: a brand new double-data plan. Verizon will be starting a promotion on November 8th, 2011, in which both existing and new customers are eligible for double data plans. Those people getting a 2GB data plan now for $30 will soon be getting a 4GB data plan for that same $30. Sound like a pretty gooddeal to attach to your brand spanking new LTE device?