Sparse columns are ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve nonnull values. Consider using sparse columns when the space saved is at least 20 percent to 40 percent. Sparse columns and column sets are defined by using the CREATE TABLE or ALTER TABLE statements.
A sparse column cannot be part of a clustered index or a unique primary key index.
http://blogs.technet.com/b/andrew/archive/2008/02/28/sql-server-2008-sparse-columns.aspx
http://msdn.microsoft.com/en-us/library/cc280604.aspx
