I find it interesting to look through the suttas, commentaries and so forth and look at the different ways in which stream-entry is represented, and in particular, what needs to be done in order to achieve stream-entry.
After the Buddha (of whom it's interesting to note that little is said about any aryan attainment prior to becoming a fully-fledged Buddha), the next aryan in the Buddhasasana was Añña-Kondañña. In...
SN 56.11: Dhammacakkappavattana Sutta
http://www.accesstoinsight.org/tipitaka ... .than.html
Gratified, the group of five monks delighted at his words. And while this explanation was being given, there arose to Ven. Kondañña the dustless, stainless Dhamma eye: Whatever is subject to origination is all subject to cessation.
Then the Blessed One exclaimed: "So you really know, Kondañña? So you really know?" And that is how Ven. Kondañña acquired the name Añña-Kondañña — Kondañña who knows.
As we see, stream-entry (at least) certainly occurs for Añña-Kondañña during the exposition of the sutta itself. Sometimes when mention of stream-entry or other attainments are mentioned at the end of a sutta, it is said that the attainment didn't actually happen then and there, but the person contemplated what was said, meditated on it, and found enlightenment. However, clearly this was not the case for Añña-Kondañña... he was not meditating, he was actively listening to a discourse.
Fast-forward several centuries to the days of the Mahavihara in Sri Lanka, and the commentaries tend to paint a very different picture on what is in fact required to attain stream entry. The classification of the vipassana-nanas (or "insight knowledges") is used to track the aspirant's "knowledges" and only at the very end of this process, is nibbana actually experienced, and with that, the eradication of the fetter of doubt, which is required in order to attain nobility in the Buddhasasana.
To me, there seems a certain disconnect here between the two examples, and I think it would be very interesting for us to discuss this, clarify and maybe find out precisely what is the best way to know what is required in order for us to attain stream-entry.