Bush's penchant for dissembling aside, I think there may be an interesting balance of powers question here that is worth discussion.
What exactly is a the policy of 'preemption' mean. A limited definition would be to attack before existing weapons of mass destruction are used. Treating terrorist attacks as acts of war by any country harboring the originators of the attack is a related issue, but not the same policy.
So, is this the definition the Bush administration is using? The answer, at the moment, is clearly no. The administration has not admitted making a mistake on Iraq and is currently claiming that they never said Iraq was an 'imminent' threat. The veracity of this aside, this and other statements from the administration make abundantly clear that even if Iraq did not pose an imminent threat, that the war was still justified.
So, what is the administration's definition of a policy of 'preemption.' I'd say that 'strategic war making' may be a better description of he administrations view of this policy. The thought behind such a policy is that it is hard to know what another country possesses and we can't afford until they use the weapons or perhaps can't even wait until they develop the weapons to attack. Other considerations are also included: is this country in violation of treaties and/or U.N. sanctions, is this country autocratic and does it possess a bad human rights record, and would the world be safer for democracy and/or the United States were we to invade this country. Many arguments for the war that weren't coming from the administration used some variation of this policy.
'Strategic war making' also has another dimension, "what is the cost?" There are good reasons for not going to war with North Korea despite the fact that in almost every way they are worse offenders than Iraq. Many more civilians, let alone U.S. soldiers would die. The question of the size of the alliance also comes into play here, the greater the commitment of allies, all else equal, the lower the price for our share and sometimes the lower the total price.
Now here's the important part. "Strategic war making" is risky business and requires a lot of discussion and thought to determine if the war is worth it. The potentially important parties to this discussion are the President, the Congress, and the American people. I believe, based on what the administration has regularly said and done, that they think the decision and responsibility ultimately rest with the President and Congress's role, at best, is to advise and consent. That is to say, the President may seek authorization from Congress, but he picks the policy for the yes/no vote and he is only obligated to share information as he deems appropriate. (The masculine in these cases should be taken to include the possibility for the feminine on some future date).
So, is this a good idea? Should the executive branch have the power to engage in strategic war making with the advise and consent of Congress? This isn't just a question about the Bush administration, I believe it's safe to assume the average administration making this choice in the future, Republican or Democratic, will be superior to the present administration.
I'd say no. Presidents and other leaders from history have regularly made bad calls when it came to 'strategic war making'. Admittedly, so have congresses and parliaments, for example FDR was more supportive of going to war against fascism than the majority of Congress was at that time. But on the whole, I think 'strategic war making' is such an awesome and terrible power that we can not leave it to one man. It can of course be argued that Congress's role of advising and consenting is sufficient a balance, particularly by looking at the regular failure of Congress to consent to recent administration's judicial appointees under a similar process. However the president already has such broad discretion in war, and will likely continue to possess it as WMDs and terrorism remain dangerous foes, that Congress's role in wars that poll fairly well, will be that of a rubber stamp.
Whether 'strategic war making' is a power the government should have at all, is a separate question, and one I'm quite willing to debate. However, if such a power is to exist, I do not think we can afford to allow it to be possessed almost exclusively by the executive branch.