Detta borde fungera:
DECLARE @StartDate DATETIME
DECLARE @EndDate DATETIME
SET @StartDate = '2012/11/01'
SET @EndDate = '2012/11/05'
SELECT
(DATEDIFF(wk, @StartDate, @EndDate) * 2)
+(CASE WHEN DATENAME(dw, @StartDate) = 'Sunday' THEN 1 ELSE 0 END)
+(CASE WHEN DATENAME(dw, @EndDate) = 'Saturday' THEN 1 ELSE 0 END)
http://sqlfiddle.com/#!3/d41d8/5707/0