Generally the user cannot decide which slots have what signal the slots run at, the manufacturer decides that; all a user can do it stick the right combination of cards in their board. EVGA has a visual guide
of all their boards available as pdf, where on page two shows which slots have a permanent signal and which have variable depending on occupied adjacent slots.
In link above, page two, show that the slots circled in green (from CPU) and orange (from X79 chipset) are permanent while the slots circled in blue (from CPU) are variable and depend on each other. They can't
all be x16 because there are only 40 total lanes in the CPU's PCIe 3.0 controller.
If PCI-e lanes are important to you, you may want to wait and see if they start using bridge chips to add them. (like the nf200 in the X58 E759 classy and 4-way classy days).
If you stick a PCIe x16 card in an x8 slot, it will run x8 and doing so it castrates its own potential, meaning throttles itself back. It won't operate 100% 8 lanes if it needed more, rather drop down to 7 lanes worth or less.
PCIe bridge chips are not PCIe controllers, they just provide a signal to the graphics card so they don't throttle back as if to convince them there is more space when there isn't really more space. At the very least there can be a performance difference from not throttling back.
The current PLX PCIe 3.0 chipset used in some X79 and Z77 boards can provide 48 PCIe 3.0 lanes worth of "telling your graphics what to run at" versus actually giving your graphics the space. A PLX chip in X79 would allow a manufacturer to brag quad x16 (48 in PLX + 40 in LGA2011 < 64 needed), but you'll still have x10 split among all four slots which is marginal against quad x8 that most X79's provide. Marginal for games with modern infant PCIe 3.0 cards, computation and folding may actually need those extra 2 lanes per slot.
<message edited by lehpron on Thursday, April 12, 2012 4:44 PM>