Microsoft is 'officially' supporting the creation of themes for Windows 7, though I mus note it is rather limited. By that I mean you can change the color of the tinted glass, the fonts, the wallpaper, etc, but you cannot change the look of Explorer, the Start Menu, etc. like you can with Stardock's WindowBlinds.

The distribution process has not been finalized, but you can click here for some existing theme downloads and limited information.