Subqueries in the SELECT Clause

Louis Davidson tries out a few query forms:

The post states that a query such as:

SELECT soh.SalesOrderID,(SELECT  C.AccountNumberFROM    Sales.Customer AS CWHERE   C.CustomerID = SOH.CustomerID)AS CustomerAccountNumberFROM   Sales.SalesOrderHeader SOH;

will, by definition, execute that subquery on the Customer object one time per row in the SalesOrderHeader table.

But is this true?

Click through as Louis tests a few variants of this using SQL Server.