I was just about to buy a new bag on the Apple Store. I had the bag I wanted open in a page in Safari, and was doing some research on other tabs. The Apple Store was working fine.
After about an hour, I came back and clicked on the "Buy" or "Checkout" button and got the "We are busy updating the store for you and will be back shortly" message. This is the Australian store, but I checked the US version and got the same message.
Could it be the release of Lion?
The rumour is that it'll launch today and it's about 3:30am in Cupertino right now. Maybe they're working on it right now. Speculating is fun.