A few enterprising developers have built an emulator for Windows Sidebar to run on Windows XP.  I'm not going to link to it because of a few things:

• We didn't officially release Windows Sidebar... yet. Think around Beta 2
• Developers who build on this platform will be disappointed b/c it is evolving
• Gadgets will be available on XP - officially (Ed. Note: I've since changed teams and understand this was the POR at time of writing however is no longer the case.)

I'm super-excited to see all the enthusiasm around Windows Sidebar but I also don't want developers to get frustrated building on the wrong platform. We have big plans in the works- when the timing is right :)