From the Office of the Pardon Attorney:

1.2 Eligibility for filing petition for pardon.

No petition for pardon should be filed until the expiration of a waiting period of at least five years after the date of the release of the petitioner from confinement or, in case no prison sentence was imposed, until the expiration of a period of at least five years after the date of the conviction of the petitioner. Generally, no petition should be submitted by a person who is on probation, parole, or supervised release.

Of course the president, and especially this president, can do as he pleases. The elder president Bush did, as did Clinton. Now it's the current president Bush's turn. However, Mr. Bush claimed early in his administration (February 2001):

...should I decide to grant pardons, I will do so in a fair way. I'll have the highest of high standards.

So does the "highest of high standards" permit the president to engage in (gasp!) executive activism? My money's on yes, it never stopped him from pardoning the turkey did it?

[UPDATE 3/11/07] A few interesting points to throw out there as we debate whether or not Libby will get the pardon and forever link this scandal to the White House.

In the comments we're directed to this article (h/t Agricola) which gives us a brief history of the politics of pardons. Looking at Bush Jr:

Bush has issued just 113 pardons during his six years in office. A flurry of pardons often comes at the end of a presidency, like they did, controversially, during President Clinton’s final days. But presidents used to issue 100 or more “clemency actions” a year, according to the Justice Department.

During his six years as governor of Texas, Bush granted fewer pardons — just 18 — than any governor had since the 1940s. One of those pardons he came to regret. In 1995, Gov. Bush gave a pardon to Steven Raney, a deputy constable who had a 1988 marijuana conviction. But Raney was later accused of stealing cocaine in a drug bust.

As governor of TX and in the wake of Clinton's pardoning of Marc Rich, Bush Jr. has been timid with his pardoning. In a 1 February 2007 interview with Neal Cavuto, Bush was pushed to define his pardon policy in reference to the boarder control agents who shot a Mexican drug dealer. In this case Bush appears to point to the JD guidelines:

“You know, I get asked about pardons on a lot of different cases. And there’s a procedure in place,” he said at first. When Bush added that he has been telling members of Congress who have contacted him about the matter to “look at the facts in the case,” Cavuto followed up: “So what are you saying?”

“I’m saying … there is a process in any case for a president to make a pardon decisions. In other words, there is a series of steps that are followed, so that the pardon process is, you know, a rational process,” the president answered.

All tealeaves from a separate case, but whatever. I guess it all depends on how he views "the facts in the case."