(added to kernel boot param - apparently idle=halt should also work).
With this both Fedora 11, and Ubuntu 9.10 Alpha 6 work. And fedora boots up quicker :)
The idle parameter has a number of values it can take, and they appear to boot the box in different fashions (halt is "cooler", or so I read on a forum - the kernel docs make it look like the mid-range C-states are not used, which is rather bizarre - a sort of all or nothing approach to CPU power management?).
From /usr/src/linux/Documentation/kernel-parameters.txt (this includes X86_64 as well):
Format: idle=poll, idle=mwait, idle=halt, idle=nomwait
Poll forces a polling idle loop that can slightly
improve the performance of waking up a idle CPU, but
will use a lot of power and make the system run hot.
idle=mwait: On systems which support MONITOR/MWAIT but
the kernel chose to not use it because it doesn't save
as much power as a normal idle loop, use the
MONITOR/MWAIT idle loop anyways. Performance should be
the same as idle=poll.
idle=halt: Halt is forced to be used for CPU idle.
In such case C2/C3 won't be used again.
idle=nomwait: Disable mwait for CPU C-states
Needless to say, I'll stay with idle=poll. Apparently this is need for all Linux kernels 2.6.24 and above.
Board found here (and elsewhere, probably):