I don't foresee them waiting long very long to get this out. They need to clear out current PS3 stock, and since the cat is out of the bag, the best way to do that is a price cut.
However, I think a lot of people would be pissed if they made a price cut, and then raised the price back up for the PS3 slim, and I don't foresee them selling the slim at a lower price point anytime soon.
Also, E3 is the biggest gaming event of the year, if Sony is really going for a relaunch of their console, I don't think they would want to miss this opportunity to announce it. It seems like the press it would garner would be equal to millions of dollars of advertising for a fall reveal. (Not that they wouldn't advertise it then anyway, but it just makes so much more sense to do a big E3 reveal.)
Of course this is just all my own armchair gamer speculation. Almost every business decision Sony has made this gen has completley baffled me, so it will likely be the opposite of what I think.