Avedon argues that impeachment will prevent bush from pardoning the many criminals in his administration:

Brent Budowski is predicting Bush will issue mass pardons of all the criminals in his administration, and I have no reason to think he’s wrong, because there is only one thing that can prevent it: impeachment.

Well, it may take more than simply impeaching bush. There is, you see, a line of succession. You will have to take out cheney before he becomes president. Then numbers 2 and 3 are democrats, byrd and pelosi, who have spent the past 7 years enabling the bush debacle.
byrd might be more likely than pelosi to not issue pardons before the next president takes office. But, really, what makes you think a president obama would not issue pardons? Bringing all these perps to trial would lead to a diminishment of the very power obama seeks.

On the less dim side of this: it will be clear that anyone bush pardons was, indeed, guilty!