The questions should have been a little more specific in it's intent. The BOL and msdn clearly says that the computed columns cannot explicitly refer columns outside the source table.

http://msdn.microsoft.com/en-us/library/ms191250.aspx

"A computed column is computed from an expression that can use other columns in the same table. The expression can be a noncomputed column name, constant, function, and any combination of these connected by one or more operators. The expression cannot be a subquery. "