I'm a big fan of the "it's none of your beeswax" option.
I also don't mind telling just about anyone that I'm an alcoholic, but I still like it to be the right time and place. A party isn't usually the place. And letting people know is a privilege, not an obligation.
I've noticed by and large that it's usually me who's most concerned about my not drinking; other people don't tend to care so much. In the three years of my trying to stay sober I think only a few people have ever asked "why?" when I said I'm not drinking.