It sounded to me like Steve was sarcastic about the 6PM thing. I think his words were something to the affect of, "I think at 6PM" with emphasis on the word "think". This is what makes me think it was sarcasm - plus the laughter of the crowd who apparently detected the sarcasm as well. Plus, on the keynote slide, when it showed the June 29 release date, even after he said the 6PM thing, it never appeared on the slide.
Just a side note - did anyone notice that Apple's stock dropped more than 3 points today? Do you think it had something to do with a disappointing keynote address (no announcement of new iMacs, lack luster Leopard demonstration (in my opinion), Barely anything about the iPhone, etc).