Se disponi di SQL 2008 puoi utilizzare il nuovo tipo di dati Gerarchiaid:
WITH Items (ItemNumber) AS (
SELECT '1' UNION ALL SELECT '1.1' UNION ALL SELECT '1.1.1'
UNION ALL SELECT '10' UNION ALL SELECT '11' UNION ALL SELECT '2'
UNION ALL SELECT '2.1' UNION ALL SELECT '20' UNION ALL SELECT '3'
UNION ALL SELECT '30'
)
SELECT *
FROM Items
ORDER BY Convert(hierarchyid, '/' + ItemNumber + '/');