Hopefully by now you have Windows 7 beta 1 running on one or more machines and you'll have spotted Themes. Themes allow you to personalise your desktop and along with tweaking gadget opacity you can create a desktop environment with candy to burn.
For more on Themes then check out Tim's post and see this tutorial on creating your own themes.
Check out Mike's site at http://blogs.msdn.com/mswanson/archive/2009/01/25/my-windows-7-theme-pack.aspx
Cheers and enjoy!!