Have you wondered what it takes to take a MacBook Air apart and what it looks like when you do? Look no further. Here's a step-by-step with high-res photos from iFixIt. The battery isn't trivial to replace, but it's definitely easier than replacing the hard drive on an iMac. I'd do it.
Unfortunately, the 80Gb drive is the largest one that will fit. I wondered about that because often Apple's top choice is one size smaller than the current leader in terms of space. I regularly crack open my new MacBook Pros before I've even turned them on to replace the drive. Looks like Apple had to go state-of-the-art here to save the 3mm that the larger drives would have cost.