Presuming President Obama's nomination of NH's Republican Sen. Judd Gregg as Commerce Secretary will be confirmed by his fellow Senators, and presuming the Democratic Governor of NH keeps his backroom "deal" and appoints a Republican to fill Gregg's vacated seat, then Democrats in the Senate ought to take advantage of some arcane filibuster math, and hold up the seating of that replacement until MN's Senate seat is properly filled.
As things stand now, while former Republican Sen. Norm Coleman is taking his sweet time in throwing everything he can think of against the wall to see what might stick, in hopes of winning his MN election contest against presumptive Senator-elect Al Franken, the Dems need two cross-over Republican votes, for a total of 60, to stymie any attempted GOP filibuster. (That math presumes that independent Senators Sanders and Lieberman both vote with the 56 currently-seated Dems, as they do on most matters.)
However, the Senate rule requiring a supermajority (three-fifths of the Senate) to shut down attempted fillibusters with a cloture vote, is based on the number of "Senators duly chosen and sworn" --- in other words, currently seated Senators.
With two seats vacant then, from MN and NH, after Gregg's departure, that would put the number of "duly chosen and sworn" Senators needed to stop a filibuster at just 59 (or, 58.8, to be precise, but since we're not allowed to count Lieberman as .8 of a human, the number needed for cloture would be 59)...