I seem to remember something about Apple's CHUD tools being able to do this. If memory serves, you can find this in the developer software included on the OS X install disc. Its under additional or optional software if I remember correctly. Let me know if that helps. I'd be interested in knowing what you find out.
Processor affinity means a process tends to run on a single processor, rather than getting randomly re-scheduled to other processors.
The way Mac OS X currently works, if a process on core 1 gets stopped (for example, it's waiting for I/O), and while its stopped another process starts running on core 1, when the OS restarts the first process it might move it to core 2 since core 1 now appears busy.
The problem with this is that each core has its own cache, and by moving a process between cores you lose all that cached information.
To the original poster: 10.4 does not do this (although I guess some people claim it can be done with special tools), but I would expect that this almost definitely a feature in 10.5, especially since the whole line of CPUs is dual-core now.