[CentOS] SMP motherboard not recognized

I'm having trouble with IBM xseries 345/dual Xeon 2.66GHz showing only 
one processor. At boot I see "SMP motherboard not recognized". Using

title CentOS (2.6.18-8.1.8.el5xen)
         root (hd0,0)
         kernel /xen.gz-2.6.18-8.1.8.el5
         module /vmlinuz-2.6.18-8.1.8.el5xen ro 
root=/dev/VolGroup00/LogVol00 rhgb quiet
         module /initrd-2.6.18-8.1.8.el5xen.img

have tried acpi=no (causes boot-hang), acpi=ht (boot hang), noapic (no 
effect), and noht (no effect)

Booting to gentoo-minimal CDrom shows expected 4 cores (dual HT) with 
cat /proc/cpuinfo, only 1 using 2.6.18-8.1.8.el5xen

Any ideas?
many thanks