that requires an invasion of Pakistan - the Paks have nukes, Musharaff is weak, it could spark a takeover of the country, a war with India, the nukes falling into the wrong hands, etc, etc.
Given the level of Pakistani cooperation so far, I can't see how any of these possible outcomes could be an improvement over what we have now. Sure, we might have gotten OBL, but what would we have been left with in the aftermath in Pakistan? Whatever problems we have in post-Saddam Iraq, Pakistan could have been far worse had we gone in there. And Saddam would still be in iraq.
We should have gone into the territories of Pakistan and into western Iran if needed. We cannot grant them sanctuary. OBL and gang are still active and very likely will finally succeed in assassinating Pakistan's leadership and reattacking this country unless they are stopped. OBL and friends attacked us, not Iraq. A huge diversion of men and material has occurred. No love for Saddam, but we need to stay on target and take out al-Qaeda and their bases of support. Otherwise this war never ends which is basically what Perle advocates. That is not in our national interest. Going into the territories makes more sense than a global war.