A good question, apart from the ambiguity surrounding the resources question. Since it's calculated at runtime, it doesn't use any disk resources, and I can't see how having a select statement along the lines of SELECT a + b AS C, <other_columns> FROM <table> would use less resources than SELECT C, <other_columns> FROM <table> when C is a computed column from a + b.

Obviously, if you're using scalar UDF's in non-persisted computed columns, there's a pretty good chance you'll be hit by performance problems somewhere down the line - as I've just discovered on one of our systems, so perhaps that was it?

Duncan