Custom Sort Order - How to not duplicate the Case statement

You could multiply the sorting key by either +1 or -1 depending on whether ASC or DESC is requested:

SELECT ROW_NUMBER() OVER (ORDER BY                   
    CASE WHEN @sortColumnName = 'Variety'
         THEN 
            (CASE f.Variety
                WHEN 'Fuji' THEN 1
                WHEN 'Gala' THEN 2
                ELSE 3
            END)     
    END
    * (CASE WHEN @sortDirection = 'ASC' THEN 1 ELSE -1 END)), *
FROM   Fruits f

Since you're on SQL 2008, you could use a CTE:

;WITH CTE AS
(
    SELECT
        CASE WHEN @sortColumnName = 'Variety' THEN 
            CASE f.Variety
                WHEN 'Fuji' THEN 1
                WHEN 'Gala' THEN 2
                ELSE 3
            END     
        END AS sort_column,
        *
    FROM
        Fruits F
)
SELECT
    ROW_NUMBER() OVER (
        ORDER BY
            CASE WHEN @sortDirection = 'DESC' THEN sort_column ELSE 0 END DESC,
            CASE WHEN @sortDirection = 'ASC' THEN sort_column ELSE 0 END ASC),
    type,
    variety,
    price
FROM
    CTE

It's not as slick as the *-1 solution for this particular problem, but it can be adapted for other situations where you want to avoid code duplication.