New screenshots and information have leaked onto the internet today revealing new changes to the Ribbon UI for the Windows Explorer within Windows 8. Currently, the Explorer uses office 2007 type UI, these new screenshots suggest that Microsoft are planning to Metro-fy the Ribbon UI.

New screenshots have leaked onto the web today revealing an updated Metro version of the Ribbon UI in Windows 8. The new changes make all the buttons more square, and blend far more easily into the Windows Explorer window.
"The RC version of the Explorer in Win8 the Ribbon interface also will be entered the Metro, the Ribbon "file, home page, share and view" button to Metro, the perfect blend with Win8 new style."
Remember to take these screenshots with a grain of salt, as they could be false.
These new changes to the Explorer is another step forward to making Windows 8 more Metrofied. The Desktop side of Windows 8 hasn't been looking like Metro at all, hopefully Microsoft plan to change this before the Release Candidate.
Source: Win8China