The only 8-core mac pro to support ProTools HD is the newer one with the "harpertown" cores, running Leopard 10.5.1 and ProTools HD v7.4.1 8-core edition.
*** THIS IS THE ONLY WORKING COMBINATION ***
Anyone running an 8-core Mac Pro and ProTools HD who does not fit the above description should contact DigiDesign support for information. They have told me that this is regarded as a high-priority issue, so expect an update sometime in the near future.
In the meantime, if you are one of the unlucky ones to own an older 8-core system, please do not report any issues to us that you suspect are to do with our plug-ins, as it is creating a considerable about of work for our support team and is delaying them from reaching customers who they are in a genuine position to help.
Please note: ProTools LE systems are fully supported are not related to this post.
"I was great engineer once, but then I took an arrow to the knee..."