The difference and relationship between the fetter of conceit and the fetter of ignorance became clear to me this morning while listening to the teaching of MN 2 by Ven. Bodhi, and I'd like to share with you this new understanding. To my understanding, the fetter of conceit is "I"/"mine" making -- regarding the 5 aggregates as "I" and the 6 sense objects as "mine" out of delusion. The fetter of ignorance is not having the panna of knowing things as they truly are -- not knowing that the 5 aggregates/6 sense objects are all anicca/dukkha/anatta, which creates the delusion of regarding the 5 aggregates/6 sense objects as permanent/sukha/atta. It's not that by breaking the fetter of ignorance a Buddha/an arahant becomes omniscient (knowing everything), but that he can see things as they truly are without delusion and attachments, thus he is liberated from the all. I don't think that the Buddha had ever claimed omniscience, and I tend to think that the omniscience later added to the Buddha is probably due to the misunderstanding of the fetter of ignorance.
While listening to Ven. Bodhi's good teaching of this sutta, I also wonder how come some practitioners could understand the Buddha's teachings (at least the 1st and 2nd noble truths, and probably also the 3rd noble truth) but could not really practice in accordance with the teachings. Could it be due to the lack of understanding of the 4th noble truth, the noble path, which might be part of the 3rd fetter (doubt)? The 3rd fetter might not only include the doubt on the Buddha/Dhamma/Sangha, but also the unclarity of the path? One who hasn't really found and comprehended the path (the 4th noble truth) and doesn't yet know what and how to practice, or doesn't practice according to the path cannot be a stream winner, to my immature opinion.
Metta to all,