Didn't manage to get this working from Windows in the end, regardless of what host/switch/etc I plugged into. (IPoIB was definitely enabled in the switch partition when I tried that)
Gave up on the windows idea, installed a new HDD, put CentOS 6.x on that, and it just worked out of the box.
It looks like there's almost definitely something wrong with the WinOF drivers available for download from the Mellanox website, when used with ConnectX cards.