The company attributes this to the falling prices of RAM and the fact that the retail channel is looking to use memory upgrades as a way to boost margin. But of course, in order to address more than 4GB of RAM you need a 64-bit operating system, which is why system builders have already started pushing 64-bit Vista over 32-bit and will likely do the same with Windows 7.
The last apparent issues with adoption are software and hardware compatibility. However, unless you are running legacy stuff you should be ok on the hardware front, and even though most applications aren't 64-bit native yet, 32bit applications should usually run just fine on a 64-bit Windows installation. I for one have already made the switch, how about you?