An article from Ars Technica's Chris Foresman states:
The problem is most glaringly illustrated by merely playing music using iTunes. Though Activity Monitor will report just 1 to 3 percent CPU load, CPU power draw will increase tenfold and CPU temperatures will hover near the safe limits reported by Intel with little or no fan activity. Users were also able to reliably demonstrate a 20 percent decrease in performance, even with something as simple as plugging in FireWire- or USB-based audio interfaces.A workaround found by some users requires using a PCIe audio card instead of the standard, onboard sound card. Several users have reported that Apple technicians are both aware of the issue and working on a solution. I imagine we will see a permanent fix when Mac OS X 10.6.3 is released. The OS upgrade is currently making its rounds through beta testing and should be available shortly. We will keep an eye on this issue and let you know if the Mac OS X 10.6.3 update contains a solution for the problem.