Is abstinence education effective?

In a recent post pointing to the continuing steady decrease in sexual activity among teenagers and young adults, Stopped Clock suggested the increased emphasis on abstinence education, something that endeared Dubya to many social conservatives, may be partly responsible for the decline:

Also, why not list the increased prevalence of sex education as a factor? Even though AIDS was at its peak in the 90s, the sex education hadn't really gotten much different than it was in the 70s or 80s. Now abstinence is a big part of the advice you hear in most sex ed classes.

The degree of causation is almost impossible to identify definitively here, but from it's emergence as a health education teaching tool in the mid-eighties through the latter part of the last decade, the idea has been on the rise, and that tracks the two decades-plus decline in sexual activity among American youth (click on the image for higher resolution):


Of course, there are other institutions--namely the religious and the parental--that have preached abstinence in varying degrees for a lot longer than it has been included in educational curricula, so I'm not sure what to think.