I don't think it's unheard of. I graduated from highschool in 1991 and we had a few boy/girl sleepovers at our house in junior high and high school, mostly after special events like our end of the year choir concert or a dance or something. It was lots of fun and my parents were very, very aware of what was going on and we knew they could walk in to check on us at any time. The rules were clear, and we had a blast. I don't remember anyone's parents having a problem with it. I wouldn't hesitate to let my daughter or son attend one, if I knew and trusted the parents hosting and the other kids involved.
In fact, I'm hosting a boy/girl sleepover at my house next Friday! I direct a theater program for high school students at our local unviersity. These are very, very serious kids who work very hard all year long - and they are a very, very tight knit group. There are 13 9th-12th graders currently in our Professional Training Program. It's become a tradition to end the year with a sleep over at my house. They watch movies, sing kareokee, and have a blast. It's the highlight of the year for them, something they look forward to all year long. And, the parents know me and trust me 100%. In turn, I know and trust the kids and I am in the living room with them the whole time. No hanky panky goes on, I can assure you. I've never had a parent have a problem with it or not allow a kid to attend.