Does Mac OS X load average reporting (via top) include processes waiting on IO?
On OS X, the kernel task accumulates IO wait time and the process accumulates CPU time managing the IO call setup and results but not the actual IO wait.

Put another way, in top or Activity Monitor, IO waiting is time when the CPU is idle. The system/kernel would take any core storage / filevault processing time needed to accomplish the IO but for the general case with a normal drive there isn't CPU overhead on IO past the normal virtual memory overhead.