[Windows 8] is all in the future. All potential, nothing actual.

Gruber makes an excellent point. Today wasn't the first time Microsoft demoed pre-release software long before RTM. I dug up a few demos for the operating system that would eventually become Windows Vista, codenamed "Longhorn". At the time these demos were quite impressive showing off technologies such as:

• Integrated Keyhole map viewing functionality (before they were acquired by Google)
• WinFS data storage and management system
• Windows Indigo messaging system

But that's all they were. Demos.

When Windows Vista finally debuted 3 years later, the user experience was vastly different than what was shown in the demos. As Tom Reestman points out, there is a huge difference between these Longhorn demos and today's demos at BUILD, being that today's demo was followed up by an actual release of Windows 8 pre-beta developer's preview. But even still, given another year of planned development, you can bet that the product you see today will not be the product you will see then.