thanks for all of the great replies. an interesting informal survey with many valid points raised. seems Sailnet'ers more often wear lifejackts than average, at least the (substantial) selection bias of replying to this thread.
i have to say, although i respect contrary argments and would not neceassarily want a law, i'm in the nerd camp with davidpm. there are many people around most cruising areas who might potentially save you, but if i'm unconsciousness, significant injury, darkness or hypothermia are around and i have no flotation, i'll just be one of those sad page two newspaper stories about some guy who got knocked off his boat and drowned.