I can waste a lot of time if an interesting question gets posed. The most recent culprit was Chris Ridd in uk.comp.sys.mac, wondering aloud if threads ever got shuttled between CPUs on a Mac. Here we go...
It certainly looks like the conditions for doing this through the API can never be met on a real Mac running xnu. All you can do is assign a thread or a task to a processor set, but not a single processor. Unless that processor happens to be the only member of a processor set. Thing is, on xnu there is only one processor set, which encompasses all of the processors. So there's no way to force your threads onto a particular processor in Mac OS X. The lack of multiple processor sets means that you couldn't have a box with, for instance, both a G5 and a Xeon running a single kernel, which is kindof sad in a way. It won't get missed, though.
It doesn't even look like there's an API to work out which CPU you're currently on. What you can do is get the port for the default processor set (the only processor set), then by manually furtling with the structs you can iterate through the CPUs (or you can ask your own mach_host_port for an array of the CPU ports). For each CPU you can get the thread queue, which you could compare with your own thread to see if it's the same. That'd be tedious, but would get the required information. Once there though, I don't see any way for the thread to move between any queues except the idle and run queues on its 'native' processor; so I think the answer is that no thread ever migrates between CPUs. I may be wrong, and am happy to entertain any information to the contrary.
Unsurprisingly with no API to do what I want, little useful code came out of this exercise. I found out how many cpus the system has, but that's it really.
Update 2006-09-10 16:25 GMT - I hate this not knowing, so I decided to play the Ask Amit card ;-).