Rafael has figured out a way to turn off the Backstage interface in the Office 2010 Technical Preview, which is codenamed Outspace.
Known only by its codename at the moment, Microsoft Office "Outspace" is one of the more noticeable changes to the new Office suite. Upon clicking of the Office button, you’re whisked away into a fancy task-oriented view that lets you manage your open document, presentation, or spreadsheet at a much higher level.
For those of us that hate change, fear not. You can revert back to the old way of doing things with a quick registry tweak. Simply navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Common\Toolbars (creating that key if it doesn’t already exist) and create a DWORD named UseOutspace. Give it a value of 0 or 1 (representing disabled or enabled, respectively) and you’re done. You’ll notice with Outspace off, a piece of the original Office button shows up. This is a bug that’ll be cleaned up as the product matures.
Good stuff. Backstage is going to take some getting used to, methinks.