Microsoft seems to be previewing the future of the Windows UI with Office 14. Take a look at SharePoint's window border. It may be that, with Microsoft flattening the Windows interface in Windows 7 (as can be easily seen in the new Explorers - the file manager and Internet Explorer), Office is to follow suit. Besides a flatter and brighter ribbon, the Aero interface is to be flatter as well.
Either that or the user who leaked the screenshot applied a special theme...
Author of The Unix Geek and The Rambling World.
What I use:
MacBook (Early 2006) with Mac OS X 10.5.4
Dell Inspiron 530 with Windows Vista Ultimate and Ubuntu 8.04 Hardy Heron (dual-boot)
iPod nano 3rd generation, 4 GB silver