I have been using Vista x64 since March of this year and have found it to be very reliable and very compatible. It runs basically everything that I care to run on it, and in fact cannot tell you of any application that I have tried that didn't work.
When I first received my Z3100, there was no driver available for it, however the Windows XP64 driver model is essentially that used by Vista 64, so I installed the XP64 driver from the HP web site (it was a signed driver) and it worked pretty much without problem. It was in June I believe (see my first posting) that I noticed that a new release of the driver was available in Windows Update and I of course installed it. For the most part, it worked fine, however I was never really able to see the custom paper types like Fine Art Pearl so that was a little bit of a frustration.
Well, tonight I decided to check Windows Update again and lo-and-behold, a new version of the driver was available. I installed it, and now I see the complete list of paper types available that I have been interested in, such as Fine Art Pearl (Less Ink) and Fine Art Pearl (More Ink), etc. I haven't done any printing yet with the new driver release, but I will over the weekend and will report back.
Based on my personal experience, I wouldn't let the scare mongers whip you too much into a frenzy over migrating to Vista. I think it is a safe upgrade at this point, and the manufacturer support for drivers is becoming more complete. I believe that the reason the statement from HP regarding Vista support has disappeared is because the issue is now moot. They provide driver updates using the normal Windows Vista Update mechanism and there really is no need to download a driver off of a website as a standalone item. If you are installing a printer on Vista, you should be able to let the printer definition wizard search the internet (i.e., Microsoft) for a suitable driver and have it download it automatically. At least that is how I would expect it to work.
Running the x64 version, I enjoy having as much memory as I can throw at applications, and I find it very responsive. For me it has been a good experience, but as they say, YMMV.