Below is a one line statement to compute last date of the month for a given input date.

DECLARE @MyDate datetime

SET @MyDate = '2012/11/25'

SELECT DATEADD(DAY,DATEPART(DAY,DATEADD(MONTH,1,@MyDate))*-1,DATEADD(MONTH,1,@MyDate))

--Output result: 2012-11-30 00:00:00.000