These are past performances, and they did that with an iMac, MacBook Pro, and Mac Pro that were very competitive performance wise compared to the rest of the industry.
Apple will not maintain its performance with the iMac using Core 2 Duo processors unless it drops the price substantially across the board. I can see the desire to hit a lower price point, but they can't just ignore performance and expect to maintain their sales figures.
I could see them doing something like this: (essentially drop the price across the board, and introduce a new high end model)
$999 20" iMac 2.66 Ghz Core 2 Duo, 2GB Ram, 9400M
$1299 24" iMac 2.66 Ghz Core 2 Duo, 4GB Ram, 9400M
$1599 24" iMac 2.93 Ghz Core 2 Duo, 4GB Ram, new video card
$1999 24" iMac Xeon L3426 1.83 Ghz, 8GB Ram, better new video card
It should be noted that the Xeon processor is 1/2 the price of the current 3.06 GHz processor.