This new experimental user interface option has appeared in early pre-release builds of Chrome 13, where being completely hidden, it will leave only the tab bar, menu button, and content area. Even the traditional forward and back buttons have shifted addresses into the tab bar, letting them end up in the top-left corner of the window.
If you want to access the URL textbox, just click a browser tab, where a floating navigation bar interface with a URL textbox and refresh button will appear after dropping down from the tab. As long as the textbox is active, the floating bar will appear, but it will slide back up and disappear after a few seconds if the textbox is not in focus with an out of range cursor.
Will this be the way Web browsers are going to evolve in the future? Perhaps, and it remains to be seen whether other programmers are going to take a similar viewpoint.