Paul Thurrott from SuperSite for Windows published some screenshots that show off the new Start button of Windows 8.1. Thurrott also confirms that you'll now be able to use your desktop wallpaper as the Start background too, but this is off by default , and that boot-to-desktop is in there, but this feature is also off by default.
And here’s a new detail that was hinted at in Mary Jo’s post: When you hover over Start button, the button changes color, with a black background and the accent color used on the flag logo. Yes, it looks exactly like the Start Charm, with similar animations.
And when you activate the Switcher app switching utility, the Start button will appear at the bottom as the Start tip does today, and has Start text in the box, as it does now.
Finally, in the Milestone Preview (MP) build of Windows 8.1, at least, there is no way to turn off the Start button. That could change.