I stumbled on this quite by accident. I use an iMac (2.8, 24", 4 gigs of ram). Like many folks using Logic Pro 8 I often get very uneven core distributions, lots of activity on one core, next to nothing on the other. I discovered that if I create and select an empty audio track before playback, this tends to even out the core loads, often significantly so. I have no idea why this works or whether it will work on multicore machines.