The Zotac board mentioned earlier is currently the only Z68 mitx board out there, and so is the only mitx SB board that can be overclocked. However, Gigabyte are bringing out a Z68 mitx board instead of their planned P67 one. Still waiting on an eta - I'm hoping to get one for a new htpc I'm planning.
I personally wouldn't buy the Zotac board - but that's just me - I'm waiting for the Gigabyte.