You cannot have two devices in two different contexts -- such as one used by VMkernel and one in pass-through -- using the same PCI slot. For example, the dual head NIC is dedicated to the VMkernel OR is available for pass-through. If you select one, the other is automatically selected as well. A dialog informs you why this occurred.
That is true for integrated devices as well as add-on devices. Specifically with respect to motherboard-integrated devices, though, there is an additional factor at play: Motherboard manufacturers may consider the silicon on the motherboard to be a non-divisible entity, and make hardware and firmware design choices which assume, for instance, that all the motherboard hardware will be under the control of a single OS for purposes of power management, resource allocation, etc. Those assumptions may be broken when any subset of the motherboard-integrated devices are taken separately and passed through to a VM, because we have no way to characterize how the passed-through devices might interact with other motherboard components. Motherboard hardware or firmware could then start interfering in unpredictable ways with a device which is now supposedly under the control of the guest OS, or the guest OS could even end up interfering with other host hardware. Any add-on card essentially guarantees hardware isolation by design; Integrated devices do not have any such inherent guarantee.
We've had plenty of reports of success with passing through various motherboard integrated devices, so my gut feeling is that the above is mostly a theoretical concern, but AFAIK this is not a configuration that we're likely to officially support in the general case.
Hope this helps!
BTW, jellyfish, I have a DQ77MK at home, although it's running Debian 7, not ESXi 
. Let me know if I can collect any info to help with your purchasing decision...
--
Darius