We've encountered an issue with Maya and the Intel Nahalem processors where Maya only detects as more cores as there are physical processor packages (as opposed to cores). A single quad-core machine has 1 processor package. A dual dual-core machine has 2 processor packages.
This causes Maya to render on only 2 cores (in a dual-quad), even when told to render on all processors. Maya is unable to auto-detect the correct number of processors, and ends up thinking the host has only 2 cores, so it uses all 2...
We have a patch for this available to customers running 5.5-0, and this patch will be part of the 5.5-1 release.
If you're experiencing this issue and are on 5.5-0, please contact Qube customer support at support@pipelinefx.com for the patched files.