Personal Technology -- Boot Camp Turns Your Mac Into a Reliable Windows PC: "You can't run both operating systems at the same time. Switching between the two requires you to restart the Mac; the operating system you're not using is shut down. That makes switching a little slow, but it also means that each operating system runs like a separate computer, with full control of the hardware. This allows Windows to run at full speed and protects your Mac files from the effects of Windows viruses."
Ahh -- so the serial multi-OS model is a feature... (the link is to the no-WSJ-subscription-required version of Walt Mossberg's column; read it for some additional features, e.g., needing to reset the Windows system clock every time you reboot).
I wonder if Boot Camp has anything to do with Avie Tevanian's abrupt departure from Apple.