I find that the best solution to comparing a datetime field to a date field is the following:
DECLARE @StartDate DATE = '5/1/2013',
@EndDate DATE = '5/1/2013'
SELECT *
FROM cases
WHERE Datediff(day, created_at, @StartDate) <= 0
AND Datediff(day, created_at, @EndDate) >= 0
This is equivalent to an inclusive between statement as it includes both the start and end date as well as those that fall between.