ESXi gives CPU_CORES error on a Q9550

I'm trying to install ESXi, but am getting a CPU_CORES error on install.  The box has a Q9550 cpu, where AFAIK the Q stands for Quad core, which makes this a pretty strange error.

I've got what I think is the latest version of vmware (5.1.0 - 799733), Asus P5Q mobo with latest BIOS, 8GB RAM, Adaptec 5405 RAID and an Intel 1000 NIC.

We're likely going to replace the CPU/Mobo in the next month or so, but I'd like to try and get familiar with 5.1.0 now, as well as transfer some load off our other 4.1.0 host.  Is there any way to skip this check, or maybe I've broken something obvious?

Thanks