Meanwhile, Judge Walton asks if Libby is even eligible for probation when his sentence was commuted before he served it (the statute assumes that supervised release follows incarceration). He's asked for briefs by Monday (Judge Walton's order and further discussion at Scotus Blog, Sentencing Law and Policy, Big Tent Democrat and Firedoglake).
The good folks at Talkleft, however, think they have two plausible answers to this: (1) the day Scooter got booked counts as one day of incarceration for time-served credit, so that satisfies the statutory pre-requisite; (2) the statute doesn't matter, because the commutation power is plenary, and the president can transmutate prison time into something less harsh (people can refuse pardons, so there is no risk that something harsher would be substituted). As a result, the statute prerequisite isn't relevant here. Details at Suggestions for Judge Walton on Libby's Supervised Release.