Best Buy is kind of doing this with their "buy back" program. If Apple continues becoming more and more predictable in terms of product release dates and pricing, I could almost see a startup doing this on their own. You buy Apple upgrade insurance, and they guarantee that you can trade in your current product for a new one as soon as the new product is released.

The only problem is that there's way to much risk to do this unless you are Apple or have a direct relationship with Apple. Then again, maybe the float would cover the risk.