In exchange, Senate Majority Leader Harry Reid (D-NV) will agree to a formal conference committee. The House will bring its partisan, one-year extenders bill to the table (complete with policy riders and pay-fors that cut programs like Medicare) and the Senate will bring the bipartisan legislation that passed overwhelmingly on Saturday.
This is a fairly minor concession for Reid. He's been on the record for days now saying he'd resume negotiations on a full-year extension as soon as the House passed the Senate bill. He's saying that instead of taking the lead on those negotiations with Boehner and McConnell, that a formal conference committee would get first bite at the apple.
Democrats have no reason to believe the conference committee will actually result in anything. For one, Republicans have already announced they will appoint to the conference committee several negotiators who have been on the record in opposition to any extension of the payroll tax holiday. Dems strongly suspect that in the end, just before the two-month stop gap measure expires, the issue will be settled in private discussions between party leaders, regardless of whether there's a conference.
After conceding for weeks, the Dems finally said this far and no further -- and actually meant it. You read that right.