Arguably, the most exciting announcement from Apple over the past few months has been for iOS 4. With all of the new features, many people are chomping at the bit for the new OS to arrive.
Bear in mind, iPhone OS 3 (released last year) took the better part of the day to hit the public. We would expect, as in the past, the update to hit the App Store around 1 PM Pacific time.
As of now, it appears that the App Store is still up and running, but watch for the pull offline as we get closer to the iOS 4 release. [Thanks to a commenter for clarifying that the App Store shouldn't go offline for an OS update.]
We will, of course, keep you updated as things progress. In the mean time, if you’ve not updated your iTunes to the 9.2 version, you’ll want to get on that task as it is required to support iOS 4.