Dual-socket motherboards are usually reserved for the top-end socket, 1366 currently and 2011 in the future. I doubt you will ever see dual 1155 motherboards, there aren't any dual 1156 motherboards as far as I know.
To put it simply, non-server CPU's do not have the functionality to communicate with each other. Opterons and Xeons designed for multi-CPU support have a specific feature which allows them to communicate with each other on a board, which is omitted in consumer CPU's and lower end server CPU's.
Edit: this is copied from a forum answer on another site, but sums it up pretty nicely.