Enabling SMP causes boot failure

I'm using 2.6.18.1 on a dual Athlon MP 1800+ machine with SMP disabled
which works just fine.  As soon as I enable SMP in the kernel, I get a
bunch of errors about nbdX sending on a closed socket, the device
mapper and the kernel fails with No volume groups found.  If I disable
Network Block Devices and enable LVM and Device Mapper, the nbd and
device map errors stop but I still have the volume group error and a
boot failure.

Any idea why?

Thanks,
Matt