The idea is rather than saying, "rates at zero til 2015", to say "rates at zero til unemployment falls to X% or inflation rises to Y%".
Of course, picking X and Y is not an easy task. Bernanke might think X=7 and Y=2.5, while Krugman might be more of an X=4 and Y=10 kind of guy.
Charles Evans is given credit for pushing this path, and Janet Yellen, the Fed vice chair seems to be a recent convert.
Obama's re-election gives the Fed a lot more breathing room to experiment with these non-traditional policies, so I give Yellen political astuteness points for holding her fire until after the election.
I still don't see benchmark forward guidance as anything remotely resembling an effective medicine to cure the economy, but it is a better form of guidance than calendar guidance, even though all the caveats about time consistency, binding future Feds, and political pressure still apply equally.