Even 600 AD. Muslims believe one of the purposes of the Quranic revelation is/was to correct the mistakes, if that is the right word, in the transmission of both the Old and NewTestaments from their sources onto the later generations, and the filling in of much material that had been, for whatever reason, excised, lost, or forgotten. That is pretty much mainstream Islamic understanding. If someone wants to dig into this, I prefer the older English translations of the Quran, such as A.J. Arberry or Yusuf Ali, especially for westerners coming out of a Christian or Jewish background.
So how come that present-day Muslims think it was Ishmael? On what authority other than an unfounded consensus constructed so as to cast doubt on prior scripture? Alex is right. You can't wait 600 years and then create a religion that on the one hand agrees to some degree with prior scripture, but on the other arbitrarily dismisses other parts of it. To do that, you have to believe a priori that Muhammed was in receipt of a new revelation that superseded prior Abrahamic ones. Except of course that in this case, the Quran doesn't shed any light on the issue. It's as interesting what the Quran doesn't say (but most Muslims probably think it does) as what it doesn't say.
It's important for you to realise that I'm not a Christian or Jewish apologist. I'm arguing about the indisputable facts about what was said in their scriptures, not about whether or not it was correct in the first instance. If you want my opinion, the whole story about Abraham and the attempted sacrifice of his son is just that -- a story, doubtless with some allegorical significance, but no basis in fact.