Before anyone get's carried away (too late) take a look at David Kopel's post over on the Volokh Conspiracy: In Anti-Republican Deluge, Second Amendment Gets A Little Damp. He's also got a similar article in the National Review Online. His conclusion is that the net losses for gun rights are only -1 in the Senate (versus -5 or 6 for the Republican party), -14 in the House (versus -32 for the Repubs) and -1.5 in governerships. A lot of the newly elected dems are pro-gun. Kopel contrasts this to the 1994 election where Democratic party losses were pretty much synonomous with anti-gun losses.
Now, most of the higher-ups in the Democratic party are antis (the leadership, committee chairs, etc.) so they can controll what bills get brought to the floor. However, I think they'll have a hard time getting anti-gun legislation through. The party has a lot of arm-twisting power, but I'm sure the new electees from red-leaning states are cognizant that any anti-gun votes will come back to haunt them next election.