For anything less than 24 hours, you can do this:
SELECT
CONVERT(varchar(8), DATEADD(SECOND, DATEDIFF(SECOND, segstart, segstop), 0), 8)
FROM (
SELECT CAST('20130906 09:28:00' AS datetime) AS segstart, CAST('20130906 09:37:22' AS datetime) AS segstop
) AS test_data
Edit: Added test data.
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.