Are you referring to the PCI-X Slots, found on some Apple Power Macs, and server NIC Cards?
I have personally not run across any 2011 mobos with that type of slot. There might be a couple of manufactures who do make a board with a PCI-X slot.
If you don't mind me asking, what are you planning on using the slot for? Do you have a PCI-X card that you want to use?
I apologize for my curiosity, I have had a few mobo's with those slots, and I always ended up using them as standard PCI slots.