ASUS ROG Xpander is simply a PCI-Express daughter card for the ASUS Rampage III Extreme. The circuit board comes with four PCI Express x16 slots and two Nvidia NF200 PCI-Express bridges. These circuits take the 32 available PCIe 2.0 lanes of the Intel X58 chipset and expand them to 64 lanes, divided across the four PCI Express x16 slots on the daughter board. In the end this enables you to connect four graphics cards with maximal PCIe x16 bandwidth.
ASUS ROG Xpander only has one real use, to enable 4-way SLI on ASUS Rampage III Extreme. Quad-SLI with dual GTX 295 or the coming NVIDIA D12U-15 works without ROG Expander. They already have an integrated NF200 chip (also known as BR04).
The reason ASUS has designed a separate daughter board for Rampage III Extreme is that Nvidia demands that motherboards supporting 4-way SLI has to use two NF200 bridge chips, and an official 4-way license key. According to our sources this adds about $100 to an already expensive motherboard.
With Nvidia's flagship GeForce GTX 480 we have seen the ways of 4-way SLI and how it can break records like no other in the right hands. This is the reason ASUS developed ROG Xpander. The number of consumers that are interested in 4-way SLI isn't big enough to justify the added cost, but the extreme users should not have turn any other way but just upgrade to get 4-way SLI support with their ASUS Rampage III Extreme motherboard.
Judging from the information we have the reason for Rampage III Extreme not getting 4-way SLI from the start wasn't just the cost. The two bridge chips does enable higher bandwidth, but also higher latencies that most non-4-way SLI systems would be crippled by. Second generation PCIe x8 offers enough bandwidth to keep most users happy.
One of few Intel X58 motherboards that support 4-way SLI from factory is Gigabyte GA-X58-UD9 and if you compare the price with ASUS Rampage III Extreme you see the extra costs we're talking about. Gigabyte's motherboard is listed for $700 at Newegg.com, while Rampage III Extreme is listed at $380.