As far as I know, the 'BBC understands' tag is a reliable one. I don't recall a time when they've said "X to sign Y" and then it turns out to be rubbish. Even in our Baptista dealings, I don't recall BBC Sport reporting that we were set to sign him; only that a fee had been agreed.
But I understand the view that nothing is concrete until on Arsenal.com. I'm sure Arsene will be asked about it in the pre-match conference - is that tomorrow? Or is it delayed because we don't play until Monday?