Well, last time I looked there were still a LOT of stores selling machines with 4GB of RAM and 32-bit Vista. Big brand names too, not little outfits. And not just 32-bit Vista, a lot of those machines, particularly laptops, have their 64-bit CPU wedded to a 32-bit mainboard chipset, physically capable of only 2^32 different addresses, which means they can't use the entire 4GB of RAM no matter what you run, Vista, Linux, or OS/2, because there'd be no physical addresses left for PCI devices. But they know users aren't technically capable enough to ask what's going on, so the advert says "4GB of RAM*" and the small print for the asterisk just says that you can't use all that RAM. You're still really buying 4GB of RAM, but to actually use it you'll need to throw the rest of the machine away. In practice such machines leave about 3.5GB of RAM accessible.
But also there is also still a LOT of software out there, particularly drivers, but also all manner of other things, that just doesn't work on 64-bit Vista. Not "it has to run in 32-bit mode", nearly everything is like that, but stuff that just won't work at all, if you're lucky you get an error, otherwise it just crashes. So I don't buy this idea that there are tons of people running 64-bit Vista and not noticing. Maybe it's happening on the dual-architecture Linux distros like Fedora, but not on Windows.