The QPI links do make it easier** to make multi socket motherboards but that is mainly targeted at workstations and servers rather than consumer desktops. Consumers hardly know what to do with their current quad core processors and would receive little or not benefit from having 16 theoretical threads. Its likely we will see a hybrid enthusiast/workstation board that supports 2 sockets much like the previous "Skulltrail" boards but it will be priced out of most peoples range and I wouldn't be surprised if they locked it down to only working with "extreme edition" (or whatever they end up calling them) Nehalem cpu's.
**easier in terms of chipset's at least. I imagine routing the traces for two sockets each with 1366 pins is a complete nightmare and costs a fortune. If nothing else that's going to ensure that multi socket boards cost a little bit extra.
From what I understand the memory support is mostly just a voltage limitation. At the moment there are no DDR3 modules I know of that can sustain anywhere near 1900mhz with less than 1.65v but chances are that will change in the coming months. The motherboard makers are probably just covering their backs for when higher DDR3 speeds at less than 1.65v become a reality.