Hang on, now. The release date for Clarksfield is September or October (according to Wikipedia). I didn't see that until very recently.
So... IF and only IF Apple uses Clarksfield in the first Nehalem iMacs (I do not believe this. I believe that they will use Arrandale), we COULD... mmmereaurghmmm... COULD. See them in September.
Of course, they would be released after the Back To School promo ended, but before Snow Leopard came out, so that's a little bit too short of a timeframe.
Oh, and with iPods being released in September in that same timeframe, the chances of a new iMac then are slim to nonexistent.
Of course, again, I don't believe they'll use Clarksfield because of the clock speed sacrifice. No consumer cares that it is quad core; they won't buy it because the clock speed sucks (Megahertz Myth), and they would be RIGHT to do so, since VERY few consumer apps would even use all four cores. More than half of those apps would run more slowly on the new than the old.