How do I perform an IF...THEN in an SQL SELECT?

You can find some nice examples in The Power of SQL CASE Statements, and I think the statement that you can use will be something like this (from 4guysfromrolla):

SELECT
    FirstName, LastName,
    Salary, DOB,
    CASE Gender
        WHEN 'M' THEN 'Male'
        WHEN 'F' THEN 'Female'
    END
FROM Employees

From SQL Server 2012 you can use the IIF function for this.

SELECT IIF(Obsolete = 'N' OR InStock = 'Y', 1, 0) AS Salable, *
FROM   Product

This is effectively just a shorthand (albeit not standard SQL) way of writing CASE.

I prefer the conciseness when compared with the expanded CASE version.

Both IIF() and CASE resolve as expressions within a SQL statement and can only be used in well-defined places.

The CASE expression cannot be used to control the flow of execution of Transact-SQL statements, statement blocks, user-defined functions, and stored procedures.

If your needs can not be satisfied by these limitations (for example, a need to return differently shaped result sets dependent on some condition) then SQL Server does also have a procedural IF keyword.

IF @IncludeExtendedInformation = 1
  BEGIN
      SELECT A,B,C,X,Y,Z
      FROM   T
  END
ELSE
  BEGIN
      SELECT A,B,C
      FROM   T
  END

Care must sometimes be taken to avoid parameter sniffing issues with this approach however.

The case statement is your friend in this situation, and takes one of two forms:

The simple case:

SELECT CASE <variable> WHEN <value>      THEN <returnvalue>
                       WHEN <othervalue> THEN <returnthis>
                                         ELSE <returndefaultcase>
       END AS <newcolumnname>
FROM <table>

The extended case:

SELECT CASE WHEN <test>      THEN <returnvalue>
            WHEN <othertest> THEN <returnthis>
                             ELSE <returndefaultcase>
       END AS <newcolumnname>
FROM <table>

You can even put case statements in an order by clause for really fancy ordering.

The CASE statement is the closest to IF in SQL and is supported on all versions of SQL Server.

SELECT CAST(
             CASE
                  WHEN Obsolete = 'N' or InStock = 'Y'
                     THEN 1
                  ELSE 0
             END AS bit) as Saleable, *
FROM Product

You only need to use the CAST operator if you want the result as a Boolean value. If you are happy with an int, this works:

SELECT CASE
            WHEN Obsolete = 'N' or InStock = 'Y'
               THEN 1
               ELSE 0
       END as Saleable, *
FROM Product

CASE statements can be embedded in other CASE statements and even included in aggregates.

SQL Server Denali (SQL Server 2012) adds the IIF statement which is also available in access (pointed out by Martin Smith):

SELECT IIF(Obsolete = 'N' or InStock = 'Y', 1, 0) as Saleable, * FROM Product