Because the Intel Beartooth Pass (S1200BT) is on the VMWare HCL for use with ESXi 5, it should really work.
Both variations of the S1200BTS and S1200BTL have two nics - an 82574L (which should be picked up by ESXi 5) and the 82579 which has been flagged by many people here as not being identified by ESXi 4 & 5. See:http://hardforum.com/showthread.php?t=1607992
There is a custom workaround for this - see: http://www.ivobeerens.nl/2011/12/13/vmware-esxi-5-whitebox-nic-support/
If you need two NIC's, then you could also get an Intel Gigabit CT PCIe1x Desktop NIC which is based on the 82574LM - less than USD$60
There are also two flavours of the S1200BT - the initial and a revised (S1200BTLR and S1200BTSR model which provides support for 3rd Generation Core i3 processors and E3-1200v2 processors.
IMHO VMWare really ought to sort out the driver for 82579 because the S1200BT is the latest Intel Entry Level Server Serverboard......and it is on the VMWare HCL as mentioned.
ESXi 5.1 released, and still no progress in this matter despite of S1200BTL being listed as supported. I carefully checked "VMware Compatibility Guide" before buying new hardware, yet despite of that I'm stuck now with the same problem...