Year of the Flood
It isn't a direct sequel, but it takes place in the same universe and includes some of the same characters (Jimmy, Amanda Payne, God's Gardeners, ETC). I just ordered my copy today, so I can't say if it's any good or not yet, but the reviews have been pretty promising so far!
Also, in an interview with the New York Times today, Atwood said she plans to write one more book dealing with the O&C universe, making an eventual trilogy. Needless to say, I'm psyched! :-)