Next time, look at the Front page, or do a quick search of the forums. You can find stuff like this thread, with the same discussion.
I'll save the OP the trouble and answer a definitive no, there is no way OS 4.0 is coming out before the summer.
Makes sense since the preview claimed it is a "sneak peak". Also, all major software upgrades have occurred during then June/July timeframe. But in just one day, we'll get a sneak peak into what the iPad can really do.