As 33girl said, it depends upon how they are conducted.
In general, I don't think that a scavenger hunt is terribly risky. And it certainly has the potential to be a fun activity.
But, as with anything, there are people who take it too far. There was a group of high school students not too long ago who were charged with felonies after being found with stolen property acquired during a scavenger hunt (street signs, etc.):
http://www.courttv.com/people/2007/0...venger_ap.html
Eek...while trying to find a link to the story above, this one turned up on the Google search. Apparently one of the highest ranking items on this list was an "unrelated human baby"
Talk about taking things too far...
http://query.nytimes.com/gst/fullpag...52C1A960948260