The high-profile the administration is giving the left-leaning outlet is a nice case of symbiosis, not entirely unlike the Bush Administration's close ties to Fox, though the president's signal that he'd been briefed on the question in advance was particularly unusual.
Not only does Ben compare Obama's relationship with HuffPo to Bush's relationship with Fox, he makes the Obama-HuffPo relationship seem even more egregious! But I don't see how. The only briefing in advance Obama had presumably received on Pitney's question was that it would be from an Iranian (a fact that the White House press office would have known from reading Pitney's blog, since he put out a request for questions*). What's more, the question from an Iranian Pitney chose to ask was a pretty challenging one:
“Under which conditions would you accept the election of Ahmadinejad, and if you do accept it without any significant changes in the conditions there, isn't that a betrayal of the — of what the demonstrators there are working towards?”
It elicited what I thought was Obama's weakest and most unsatisfying answer of the presser. I can't think of any time a question from a Fox reporter produced something like that from Bush.
Update: According to Michael Calderone, the White House actually reached out to Pitney in advance and asked him to solicit questions from Iranians he could ask at the presser. So there was obviously some coordination, but so long as the White House didn't pick out which question Pitney asked (and if they did, they did a bad job of it from Obama's perspective), then I don't see a problem.