The offer begins tomorrow (June 26th, 2009), and continues while supplies last, or July 11, 2009, whichever comes first.
When you pre-order, you’ll get your copy quickly after it’s available (October 22nd! for General Availability), and you’ll get it fast and save while doing that! Pre-order a Windows 7 Home Premium Upgrade for $49 or a Windows 7 Professional Upgrade for $99 at Microsoft Store. That's about half off the estimated retail prices.
More information is available at the Windows team blog.