Circumstantial evidence of a criminal conspiracy to obstruct justice can include evidence of motive and opportunity, especially the timing of particular acts.
So, for instance, if the government is prosecuting an individual for money laundering or tax evasion, evidence concerning the timing of money tranfers can be quite significant, even if there is no direct proof that the money was used for an illegal purpose.
An article in this morning's NY Times discusses the legal basis of Bush's communtation of Scooter Libby. As one expert said, "there is none." In fact, as the article notes, the SCOTUS just decided a case on very similar facts against Victor A. Rita and upheld the sentence (33 months to Libby's 30 months). In other words, they found that Rita's sentence was NOT excessive.
The article also discusses the timing of Bush's commutation. Why now? I believe the timing of this decision is good evidence that Libby and Bush made a deal.
Follow me on the flip for some further thoughts on this.