I talk with one of my colleague about the old days of 16-bit Windows. When we talk about that in Windows 3.0, you can supply command line switch to "win.com" to instruct it to start in real mode, standard mode (protected mode) or 386 Enhanced mode (virtual real mode).
After the conversation, I thought about why Microsoft doesn't release a OS favour that you can choose to boot into 32-bit or 64-bit mode in boot menu. Sure I can choose to dual boot 32 bit and 64 bit Windows (since it neither violates per-seat nor per-device rule, licensing shouldn't be matter), but it's really painful to have all the applications like MS Office or so on installed twice.
Most 32-bit applications can be run on both realm. 32-bit Windows application takes more memory to run in 64-bit Windows, so meanwhile I can run my daily activities in 32-bit side, then switch to 64-bit when I have some 64-bit binary to run.
Drivers shouldn't be an issue because of the seperation of sysnative and syswow64 folder. If we think about the old days when Vista, the first widely supported 64-bit Windows, is just released and most 64-bit drivers weren't quite stable, it'd also give the chance to users to switch back to 32-bit Windows for a while and wait for driver update when they have driver problems. The switch from 32 bit to 64 bit would be much smoother this way.
It's too late to bring this on, considering Windows 7 has released it's beta, and it's supposed to be the last Windows OS that have 32-bit version. Just curious that whether the decision makers in Microsoft had considered about this kind of idea.