I'm all for the idea of having a computer than can run any OS on the market, but I don't think dual-booting an Intel Mac is the answer. I feel a better solution is for Apple to provide a Rosetta-like translation layer (think WINE for Mac OS X) in which Windows apps can run natively. Just being able to boot your Intel Mac into WinXP does nothing for the monumental security, virus, and spyware problems that plague the Windows world, almost all of which target the OS (or applications tightly integrated into the OS).
If Apple really wants to offer a competitive alternative to Microsoft Windows, just think how attractive a computer would be that could run natively every single piece of software on the shelf (without having to reboot into a different OS which is very consumer unfriendly), and also without Windows' security problems.
Just as Microsoft is gearing up for a Windows Vista launch this summer (or Fall as the date keeps moving), Apple could really make a splash by announcing at WWDC their next version of Mac OS X with this kind of translation layer built in.