While 64-bit processors and operating systems have been out for a while, several factors have held up adoption of 64-bit computing - such as driver compatibility issues and the mere fact that there wasn't a lot of need for it. Thus, 32-bit systems have pretty much remained the norm among consumers. But as the benefits of 64-bit systems start to outweigh the drawbacks, Microsoft believes a big shift is now on to 64-bit Windows Vista PCs.
According to a post by Chris Flores on the Windows Vista Team Blog, the proportion of all US machines with Vista running the 64-bit edition has tripled in the past three months while worldwide adoption has more than doubled in the same time frame. Microsoft's claims are based on download figures from Windows Update, though the company isn't giving any hard numbers.
Among the factors leading to the shift appear to be the fact that 64-bit Vista machines, unlike 32-bit systems, can directly address more than 4GB of memory - an amount rarely needed in 2005 when Microsoft released Windows XP in 64-bit form. Also, 64-bit drivers are required for WHQL certification in Vista so hardware compatibility has become less of a problem nowadays.
I for one have been running the 64-bit version of Vista on my main desktop PC since early June, with (almost) no issues at all. The real benefit, however, will come when more 64-bit optimized software is available and a few key ones - such as Adobe Lightroom and PhotoShop - are expected to be released this fall.