I have spoken a LOT (too many times to link back) over the last year or more about getting your applications to work on Vista. When I ask for a show of hands to see who has tested their apps on Vista, I typically get less than a quarter of the room. When I ask why I hear things like "a copy of Ultimate is too expensive just to test with" or "I don't have a spare machine with the horsepower to run Glass" or even "I don't have a spare machine".
Fear not. How about a FREE evaluation VHD image of Vista, that you can run with the FREE copy of Virtual PC on the machine you have now? It might be slow, it might not do Glass, but you can find out for FREE if your app even runs, if it works under UAC, what happens if you put a manifest on it, and so on. Come on, what's stopping you? Jean-Luc David of Microsoft Canada has all the links for you.