If the 64bit apps available to run on Vista64 were in the same ratio that would be simply capital, unfortunantly they're not, maybe you should send this graph to software developers, now theres an idea.
So true, but software developers tend to write the software for the majority of availability, and even though Vista64 sales have improved significantly through the first of the year, Vista64 is a minority in the overall scale of operational operating systems out there.
True, if you're limiting yourself to the moment. But Development cycles are still measured in years, and - if the trend for Puget Sound is representative of the industry as a whole - at this stage in the game, a software company would be foolish to not provide support for x64. We know very well that doesn't mean you have to code in 64 bits, since XP/Vista/Win7 x64 all run 32 bit programs. But you do have to QA, Support, and Bug Fix your 32 bit code so it runs properly in both.
If I may play both sides, though: Puget Sound is a small~ish custom builder, and they cater to gamers for a large proportion of their business. Address Space realities being what they are, if you're going Crossfire/SLI you almost *have* to go with a 64 bit OS in order to have enough addressable RAM to run demanding games. This little fact of personal computing life is reasonably well known among gamers, and certainly fully understood by any systm builder worth their paycheck. So it isn't surprising that x64 would be well represented in Puget's numbers, perhaps over represented.
HOWEVER: What's more telling to my mind is that OEMs like Dell, HP, and Gateway are providing x64 versions of Vista as their standard OS. Gateway is now 100% Vista 64 on the desktop, for example, as well as using Vista 64 on most of their lappys. Same for Dell: Their Desktops are available with Vista 64 as the standard OS on all but their cheapest PC's. HP aren't quite there: They provide Vista 64 on machines with more than 3GB of RAM only. What this all means is that 64 bit computing is now in the hands of Mom, Pop, Kids, and Gramma - By Default. The days when 64 bit was "just" for hard core power users and gamers has passed.
Granted - 64 bit PCs remain in the minority when you count current numbers of deplpoyed machines. I'd opine that's mostly due to the usual 4 to 5~ish year replacement cycle, rather than any significant hurdles needing to be solved before widespread adoption can take place. One of the last things I can think of off the top of my head being the day when 64 bit Flash at last moves from Beta to Production,