Hi Guy.
Looks like it may be vour Virtual Connect Firmware version.
The HP Flex-10 and Flex-Fabric Supportability list shows you need Adapter Firmware 7.2.54 and Virtual Connect Firmware 3.50 for the bnx2x driver version 1.72.54.v50.2 on ESXi 5.0
http://partnerweb.vmware.com/programs/hcl/ESX_Flex_config.pdf
3.15 is pretty old firmware but then again with it often being an unstable upgrade process, VC firmware upgrades are not normally done until you need to!
Cheers,
Julian