George Voinovich (R-OH) is retiring, joining Kit Bond (R-MO), Sam Brownback (R-KS), and Mel Martinez (R-FL). To replace him, Republicans seem likely to coalesce around George W. Bush's former Budget Director and Trade Representative. In Ohio. No, seriously. This will probably end up as the Democrats' best pick-up opportunity in 2010, though the likely Chandler-Bunning "contest" may put up a good fight.
As Chris Bowers points out, retiring GOP Senators are more likely to vote with Democrats, regardless of their prior ideological bend. Voinovich will no longer need favors from Mitch McConnell or the Republican donor base, so he no longer needs to hew quite so closely to the party line. He's also signficantly more moderate than any of the other retiring Senators, so his office really needs to be on speed dial at the White House of Legislative Affairs.