TrustedReviews posted a review today of the Asus A8R-MVP CrossFire Motherboard

Rather than using a small PCB in a slot on the board to change between single and dual card mode, the A8R-MVP uses a small piece of PCB that slots in to the top slot. This disables this slot and transfers the bandwidth to the lower slot to make this a full x16 slot. It’s a simple solution but it works well. The downside is that you can’t use the slot at all in single card mode, something most SLI boards allows you to at reduced bandwidth.

Asus A8R-MVP CrossFire Motherboard Review