Form your result set then use it as the source for the final query where you can format your data.
SELECT
CONVERT(VARCHAR(20),OrderDate,101) AS OrderDate,
TotalQty
FROM
(
SELECT
CAST(CONVERT(VARCHAR(20),OrderDate,101) AS DATETIME) AS OrderDate,
SUM(Qty) AS TotalQty
FROM
Orders
WHERE
CAST(CONVERT(VARCHAR(20),OrderDate,101) AS DATETIME) between '05/29/05' and '06/10/05'
GROUP BY
CAST(CONVERT(VARCHAR(20),OrderDate,101) AS DATETIME)
) AS subqry
ORDER BY
OrderDate
Speed wise, I think kubben is correct. I would remove the CAST/CONVERT in the WHERE clause of the subquery with a little intelligent manipulation of the date ranges.