Yes, I agree - Apple needs to release these dual core PMs sooner rather than later for that very reason. Another reason is simply one of $$$ - Apple would need adequate time and adequate sales to recoup their R&D costs for the systems to be profitable and not take a hit as a result.
I think a full year would be acceptable for most people, but anything shorter than that, and as you say, Apple might be pushing their luck with consumers. Let's hope we see these babies sooner rather than later.
And after all this dual core anticipation, wouldn't it just be absolutely hilarious if Apple's next PM update was simply to the infamous 3 GHz G5 that was promised 2 years ago? 😱 😎