Assuming that Lawrence O'Donnell knows what he's talking about when he says Rove was the one who blew Valerie Plame's CIA cover to every reporter he could find, here are the maximum downside and upside that I can envision:
Downside Scenario: White House and Republican media spin machine successfully sell the idea that the Plame outing was not a big deal (it's not like she had infiltrated al Qaeda; everyone already knew she was CIA, etc.). And not only that, but all the left-wing excitement about this is just further proof of how hysterical and out of touch with reality liberals are, that they would try to take down a great patriotic American like Karl Rove because they're resentful and jealous of his success at ushering in a Golden Age Of Republican Dominance. Oh, and no-one notices that Ann Coulter is nominated to replace Sandra Day O'Connor on the Supreme Court.
Upside Scenario: The Republicans are unable to convince a majority of Americans that outing a CIA agent is doubleplusuntreasonous, and Rove is unable to wriggle out of conviction for treason (or at least perjury). More importantly, as the story of a massive media cover-up surfaces, the myth of the "liberal media" is destroyed once and for all, and news organizations are forced to fire all their Republican mouthpieces and rededicate themselves to reporting the truth, in a desperate attempt to restore their shattered credibility. Without the media on its side, the Republican Party implodes, leaving nothing behind but a foul-smelling stain on the Congressional Record.
Of course, being a pessimist, and seeing how things have played out in the past, my money's on the downside scenario or something close to it. My sense of the past history of Plamegate is that the coverage has been sketchy, and the average American really doesn't see what the big deal is. I mainly mention the upside scenario because I believe that some kind of cleansing, cathartic scandal within the press corps itself, followed by a massive backlash against media corruption and complicity, is the only chance that we have of ever having a truly fair and balanced press in this country. And this story marks the first time that I have seen even the tiniest hint of a glimpse that it could happen.
The Democrats need to get better at playing the "corporate media" card every time a Republican scandal comes to light without media coverage, and every time a Democratic "scandal" gets breathless 24-hour coverage, only to be quietly dropped when it turns out to be bogus. As long as the media is firmly in Republican hands, Democrats and democracy don't stand a chance.