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It could be at the same event; Apple's been known to surprise.

Of course, if the MBP are just a speed bump and upgrade to Core 2 Duo (which they probably are), it will probably be a silent update.

I think the only reason the iMac update would happen at a special event would be the intro of the new screen size.

The MBP could be updated on any random morning to a Core 2 Duo.
 
Hopefully they'll be Conroe iMacs and not Merom, I see no problem in putting Conroe in iMac and that would (finally) differentiate iMac from the laptops.
 
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