It's correct that Joachim Gauck managed to swing more than just a couple of votes from the CDU. But people are quick to forget that the members of the Left would rather cut off one of their hands than to vote for Gauck.
You made the point. The SPD would have to find a candidate that has the votes of the Left party behind him, and helps getting some "traitors" from the government benches. Very difficult.
Anyway, I don't think Wulff will resign.
Which gives the SPD attack-ad fodder in advance of 2013.