Modern presidents are never challenged from their base, always by the people who didn't love them going in. You're not supposed to get a serious primary challenge from the people who loved you. But that's the talk of what may happen with Mr. Obama.
I think this is still a lot of political thumb-sucking, personally, and the reason is found farther down in the article:
The Democrats' problem is that most of them know that the person who would emerge, who would challenge Mr. Obama from the left, would never, could never, win the 2012 general election. He'd lose badly and take the party with him. Democratic professionals know the mood of the country. Challenging Mr. Obama from the left would mean definitely losing the presidency, as opposed to probably losing the presidency.
Most of the things I read (from news sites and blogs, mainly) indicate that liberals are unhappy with Barack Obama, but the alternative from the GOP (Sarah Palin is always the boogey man) scares them enough to stay loyal to Obama. Republicans angry with the GOP, wanting to "teach them a lesson," stayed home in 2008. They didn't vote for the opposition. I can't imagine liberals wanting to destroy their party over DADT or tax cuts or the public option.