I'm coming a bit late to this discussion, but what the hell:
On a pre-emptive multitasking operating system, the program with the highest priority has the CPU. It has to 'release' the CPU to other apps by going into an idle mode (normally accomplished by waiting for an event).
This isn't true - it WOULD be true of a cooperative multitasking system, but not of a preemptive one. You can verify this by writing a program that just sits in a loop. You can still alt-tab away from such a program, which proves that the operating system can preempt a running thread that is in a loop. The "preemptive" name tag means that the OS will do exactly that.
Having said that, there is room for improvement for Riftrunner over DivDiv. Many games take very low CPU (<3%) when tabbed out.
Mind you, it isn't such a problem nowadays with XP and a hyperthreaded P4. If I alt-tab out of DivDiv, it sits there taking exactly 50% of the cpu, something which I assume is achieved using Intel's much-vaunted hyperthreading.
