Everyone loves getting a deal, particularly on expensive tech devices and even moreso in the current economy. Wireless providers are well aware of this and have adapter their pricing over the years to offer the best discounts they can while keeping the deal to their advantage - giving away a free phone when you sign a 2-year contract, for example. Offering mail-in rebates is another popular technique used by companies like Verizon and Sprint; they can advertise a super-low price but the burden to take advantage of it falls upon the customer.
Last September Verizon switched from sending out standard paper checks for rebates to sending out pre-loaded Visa debit cards, a move that has been met with mixed reviews - on one hand, you can use the money right away, but you can't simply deposit it into your checking account like you can with a paper check. Now they have announced another change to their rebate policy, but this one will probably not be as popular: effective immediately, Verizon will be requiring customers to include a copy of their signed customer agreement in addition to the receipt and original UPC to qualify for the rebate.
If you purchase your Verizon data card or phone from a reputable store or online retailer (like 3Gstore!) this should be no problem, since you automatically receive a copy of your agreement with your receipt and device. It's just another copy to make and another item for your checklist!