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It's certainly going to be interesting to see what they do. I wouldn't be suprised to see them wait for 2.53GHz and 2.66GHz Nehalem mobile quad cores. I also don't think they would lose much business by doing so assuming we'd see such processors being suitable for a Macbook Pro by Q1 2010.
2.53 GHz is 45 W so it's technically possible in the iMac. 2.67 GHz would likely be 55 W so it's also technically possible in the iMac.

Even if they get the quad core speeds up by then, won't the dual cores have higher GHz by then too and the quad cores won't have actually caught up?
That's true. The thing is that quad-cores would get cheaper and cheaper and displace the dual-cores. Apple would most likely switch to quad-core in the iMac, MacBook Pro, and MacBook when their entire lineup can be quad-core. The current MacBook Pro uses $240 and $31x dual-cores, with a $530 option.

Code:
       H2 2008: Penryn  H1 2009: Penryn  H2 2009: Nehalem    H1 2010: Westmere (estimate)
$1038      2.53GHz 45W QC                Clarksfield 45W QC  45W QC
 $851           2.27GHz 35 W QC          Clarksfield 45W QC  45W QC
 $851  3.07GHz 45W DC
 $530                                    Clarksfield 35W QC  [B]35W QC[/B]
 $530  2.80GHz 35W DC   2.93GHz 35W DC   Auburndale 35W DC   35W DC?
 $31x                                                        [B]35W QC[/B]
 $31x  2.53GHz 35W DC   2.67GHz 35W DC   Auburndale 35W DC   35W DC
 $34x  2.53GHz 25W DC   2.67GHz 25W DC   Auburndale 25W DC   25W DC
 $240  2.40GHz 25W DC   2.53GHz 25W DC   Auburndale 25W DC   25W DC

The sad thing is the quad cores can (and do if you look at the Xeons) run as fast as the dual core CPUs. I think I have said before, Intel can probably clock these chips pretty close to 4GHz on air if they felt inclined to. Core 2 just scales that well.
Penryn quad-cores are just 2 dual-cores so they can clock the same. It's just that the quad-cores would have a much higher TDP.
 
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