Well, I was very excited about getting the Vista sidebar to work on XP, until I started playing with the Gadgets.
It turns out the patched version of the Sidebar executable is from a early beta version of Vista. The unfortunate point about that, is that the Gadgets that work with the XP version are vastly different than the Gadgets for the released version of Vista. That means all the gadgets that you can download don't work with the XP version. It also means that all the information about developing Gadgets for the Sidebar is relevant to the Vista version, not the XP version.
To detail some of the differences:
In the early release, Gadgets are not even called Gadgets, they are called Parts. Parts and Gadgets are very similar, in that they both are a zip file containing what amounts to a mini-webpage which is loaded and interpreted by the Sidebar.
In the XP version, a Part is a zip file with the extension changed to .part, in Vista the extension is .gadget. In XP, the directory where those files are stored is %userdir%\Parts, in Vista it's %userdir%\AppData\Local\Microsoft\Windows\SidebarGadgets.
Parts require a Manifest.XML file, while Gadgets require a Gadget.xml file. The contents of those files, while containing nearly the same data, use different names for all the tags, making them not compatible.
Beyond that there are a number of other subtle differences, as well as a generally limited set of functionality in the XP version, as compared to the released Vista version.
So, that poses a question? Considering the large amount of people who aren't interested in upgrading to Vista, due to performance or cost issues, or just not wanting to uproot and start again, is it worth my time to develop for this patched version of the Vista sidebar? Is there a substantial user-base that would benefit from having more cool Gadgets, I mean Parts, to run in their XP Patched Vista Beta Sidebar?
Perhaps that's a bigger niche than one would initially imagine.
Well, until I have an install of Vista to run the release version, providing a development environment for generating good Gadgets, I may just amuse myself with by playing with potentially pointless Parts.