As several readers pointed out, preemptively scheduled tasks is what OS X will provide. The current Mac OS is a cooperatively multitasking system. If it were preemptive there would be no more modal dialogs as all processes would be able to take some portion of the processor time. Cooperatively tasked processes seize the processor and only give part of the time when they want to, and that's exactly what the Mac does now. This reporter got the terminology reversed and is profusely sorry.
Threads are within a process and are subordinate to it in terms of the time they are allocated.
I think the basic model is preemptive, and if a programmer wants cooperative threads that must be programmed explicitly. And about current Mac OS' threads well, they can be cooperative or preemptive, but preemptive threads are somewhat restricted in what they can do. They cannot call all system calls, because the current Mac OS isn't re-entrant. X should be completely re-entrant so any thread can call any system functions they like. On the same topic, Greg Weston, who has written applications for both the Mac and Be platforms, says one major benefit to a threaded design, once developers become comfortable with it, is that design itself tends to be cleaner than the same process with a non-threaded design.