I'm actually reading a book now on the arab conquests and the author claims that most arab cavalry dismounted before fighting their Sassanid and Byzantine foes. Also a good portion of the Arab warriors did wear chain mail though their horses were unarmored. There are accounts of Persian archers slaughtering Arabs in droves at the Battle of the Bridge while at the same time being relatively ineffective at Quaysidah (sp?). So the Arabs were not necessarily as lightly armored as we might be led to believe. At least the upper classes. The Bedouin were probably unarmored.