The Bush administration has signaled that it won't be necessary to issue pardons for officials involved in harsh interrogation, even while the incoming Obama administration won't rule out an investigation. White House officials believe that legal opinions written by the Justice Department clear interrogators of any wrongdoing.
In September, Joe Biden said that the Obama administration would pursue appropriate charges against Bush officials not out of vengeance, but out of the need to preserve the notion that no one is above the law.
If Bush grants a blanket pardon to top officials, the public (and history) might consider it an admission that criminal activity plagued his presidency. Even so, do you expect to see preemptive pardons, for crimes such as torture or illegal wiretapping, before Bush leaves office?