I have the exact same problem, ie stacktrace is identical. My machine is also a Mac Pro. 2.6.20-12 worked. 2.6.20-13 renders the system unusable.
-- Kernel 2.6.20-13 acpi bug https://launchpad.net/bugs/96480 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
