Yup, the US store is down. Couldn't tell you why just yet, but if you really wanted to buy that new MacBook Air this evening, you'll have to wait until it comes back up. When we see something new, we'll post about it here -- or just stay tuned to the comments on this post!
What could it be? A Mac revision? A revamped cinema display? The rumored dead white iPhone? Your guess is as good as ours.
Update: The store is back up, with no obvious changes. Looks like it was just a maintenance update, which wasn't all that surprising since it went down on a Saturday night.