There's an "interesting" post over on Neowin about Gadget development on Windows Vista.
In general I agree with the post, however I think that the author has jumped the gun - Gdget development is very young, it doesn't have IDE support yet - but you can bet there is a team buried away in Redmond somewhere working on IDE support and a JavaScript debugger (that works).
From my small play with Gadgets I know it doesn't feel ready yet - but time will fix that I'm sure.
If anyone has done any Gadget development I'd be interested to see what they have to say.