When you buy a print WotC D&D product, you receive a promotion code that provides 3-6 months of DDI subscription credit, depending on the cost of the item.
Interesting proposal. How about a reverse approach, when you subscribe for a year for the first time, you get a free player kit of some sort.
This would let them promote D&D more as a service than a product. I know that's not what the community is used to, but for all intents and purposes, a steady flux of material and updates is more of a service than a product anyway.