There was one that was in the Dallas area a couple years ago.... they did get a lot of attention. I guess a lot would have to do with the way any encounters would go.
In theory, I would not have a problem with having a threesome with a mother and daughter. It would largely depend on how attractive I thought the pair was. The aspect of the relation doesn't bother me. I'm sure that there is a market for it. The "taboo" of being with a woman and her daughter at the same would likely attract as many men as it would repel. This isn't something that I personally would seek out though. I would be more inclined to seek out a threesome between two sisters. Although that isn't really on my list of things I want to do either.