A lot of people are worried about a gun grab, where the federal government takes away your firearms by force. I think this is overblown a bit.
I fully believe that a new version of the Assault Weapons Ban will go back into effect. Banning all imported military-style arms. I fully expect a ban on all new non-LEO, non-military magazine sizes to be 10 rounds or less. Perhaps to go so far as California's fixed magazine requirement. That's about as much as he could ram through congress and the senate before the 2010 elections. I do not see him being able to do more than that. Although the hardcore democrats would like more, he has to split his time with other projects, like working on the welfare state, righting all the "wrongs" of the past administration, dealing with the financial crisis. He won't have time, and I doubt he will try to take on the Gun Lobby, the NRA, and the bulk of the middle class during his first term.
I say the middle class because as crime increases, more and more from the shrinking middle class will buy guns to "defend" themselves. Without training, whatever they buy is useless, but they won't want to give up their protection. I might be off base with this part, but unless martial law is declared, I don't see a gun grab happening.
His second term is a different story. If Obama maintains a Democratic Senate and Congress, I fully expect a gun grab of some kind. I also expect a lot of situations to come up like Ruby Ridge and Waco. I expect a lot of people in law enforcement and civilians to die from it.