Froomkin:

We know, for instance, that Cheney was the first person to tell Libby about Plame's identity. We know that Cheney told Libby to leak Plame's identity to the New York Times in an attempt to discredit her husband, who had accused the administration of manipulating prewar intelligence. We know that Cheney wrote talking points that may have encouraged Libby and others to mention Plame to reporters. We know that Cheney once talked to Bush about Libby's assignment, and got permission from the president for Libby to leak hitherto classified information to the Times. [...]

It's true that the Constitution grants the president unlimited clemency and pardon power. But presidents have generally used that power to show mercy or, in rare cases, make political amends -- not to protect themselves from exposure.

There is so much more that he hits on, but this was such a nice, clean synopsis, I thought I'd share.