Obama only did so well in South Carolina because the African American turnout there was really high and he got a fair share of the white vote, although not nearly enough to carry the state. I think in order for Democrats to carry South Carolina (and all Southern states in general), they're going to have to do better among white voters but since they make up the base of the Republican Party (Southern whites), I don't see this happening anytime soon so I'd say DeMint is safe, unfortunately. The huge African American turnout won't be there in 2010 like it was in 2008 so that also adds to the probability that DeMint will retain his seat. Also, he was the #1 most conservative member of the U.S. Senate and was one of two who voted against Hillary's confirmation as Secretary of State (the other being Diaper David Vitter) - for that reason alone, he should go. 
Statewide races are different than Presidential ones. Inez Tennembaum nearly won in 2004 over DeMint, while Bush carried the state by 18%. I'm not saying he will lose, but its not a cut and dry case that he wins handily. He's popular in the more conservative upstate, but here in the lowcountry, he isn't invisible. Of course, it depends on who the Democrats put up. Just because Obama did poorly with whites in SC doesn't mean that no Democrat can get 30% of them, it just means that Obama could not.
