The reason Intel has given (or I should say something like "has let itself become the fact"), or the reason that Intel is trying to sell us, is that the Yorkfield models concerned by the delay are all affected by a bug which, in extremely rare cases, would result in overall system instability, crashing issues, performance loss, and such things. But something is very fishy about that. Because Intel has released the QX9650 already, which logically should also be affected by that "bug". And so far, no one on the market that we know of (mostly the reviewers) has reported anything even near such a bug.
Personally I say that Intel is creating disinformation.
They probably want to sell more of their over-priced Extreme model for one, two or even three extra months for profitability's sake. They are a huge company, and their main goal isn't to please their consumers by releasing everything they can immediately as soon as possible. Their main goal is to make money. And if they can let the rich consumers around buy more of the Penryn-related hype, then those consumers will not wait those extra months, because a number of them are probably impulsive consumers, so they will go out and buy a QX9650 instead and be done with it.