I am sort of the adventurous type I guess and so I have been running Vista 64 for about 4 months now with pretty good success, actually. Pretty much everything I have thrown at it (application wise) has run without a problem.
Prior to getting the Z3100, I had an Epson 7600 that had decent drivers, but I live in a very dry climate and I ended up wasting probably half my ink due to clogging issues, so I was very interested in the Z3100 as a solution.
When I received the HP, I installed the Windows XP 64 Z3100 driver onto Vista 64 and it worked quite nicely for the most part. Mind you, I had been willing to install a VMWare client XP to just run the driver for the HP, if that had been necessary, so it was a nice surprise when I found that Vista happily accepted the XP 64 driver (to give credit where due, on HP's web site page regarding Vista support, it does make this easily overlooked hint).
So a couple of weeks ago when the new drivers were released, I eagerly searched for either an XP 64 or Vista 64 version that I could install, but only to be disappointed. Just plain XP drivers and MacOS. grrrr. Looking further, I start to read the Vista support page on the HP site in more detail, where it pretty clearly states that their target for supporting Vista in all flavors is July 2007. OK, I can live with the status quo for a couple of months, at least I know what HP's intent is and there is hope.
Last night I was doing some maintenance tasks on my PC and so I went into Windows Update and there is a message indicating that updates are available; I click on it and, lo and behold, an update for 'HP - Printing - 'HP Designjet Z3100 24in Photo'. OK, gotta install that baby!! click! click! Installation goes smoothly. I open the printer preferences dialog and view the About Tab:
HP Designjet Z3100 24in Photo (61.71.341.0)
Run a quick print cycle through QImage, everything is great! HP seems to have delivered current support for the Z3100 on Vista 64!! Thanks!!