Fact: Silverlight is cool
Fact: a Vista Sidebar is a nifty device
Conclusion: a Silverlight Sidebar Gadget would be awesome.
Having stated the obvious let's get to realizing one, and let's take the short route:
Ioan Lazarciuc has blogged about creating a sidebar gadget using Silverlight:
http://www.lazarciuc.ro/ioan/2008/08/02/creating-a-vista-sidebar-gadget-using-microsoft-silverlight/
He builds on Tim Heuer's templates:
http://timheuer.com/blog/archive/2007/03/11/14010.aspx
This is enough to get you started. So install the templates and get ready: next time I'll be writing about a SilverNewton Silverlight Doodle that will be sitting in your sidebar and will demonstrate gravity is still going strong!