Aw, I just bought a MacBook last week ...
When you purchase a new product from Apple (either through the online store, or though a brick-and-mortar store) you have 14 days to return the product. With this 14-day policy, Apple also gives you price protection. From my understanding this covers both pricing decreases as well as new products coming out.
When I took my MacBook Pro to the Apple Store, they immediately looked at my receipt and asked if I wanted a new one. I thought, "This can't be this easy, can it?" There is a caveat with replacing your old machine with a new model: the Apple Store might charge you a "re-stocking" fee. This is a 10% fee based on the initial purchase price. I questioned them about this and they told me that I would be getting an upgraded product. However, I was able to get my re-stocking fee waived after talking with the manager for a little while.
The Bottom Line
If you bought a MacBook/Pro/Air in the past 14 days and want to upgrade to one of the newer models, it definitely wouldn't hurt to call up your local store (or Apple online customer service at 1-800-MY-APPLE) and talk to them about returning. Don't be surprised if they tell you about the restocking fee, they told me the same story until I took it into the store. Unlike most computer companies, Apple does try to make the situation right, and most of the time, you come out on top of the matter.