I went in November 2011, I asked if it was ok when I made my reservation and they had no problem with it. I detected the swimming hole at the slide and various other parts of the park. I'm going back again in 3 weeks and can't wait.....
Update:: I just returned from
Turner falls 11-27-12
They still allow metal detectors, in fact the workers in the park said people use them there pretty often. My family and I stayed 3 days and mostly hit the Blue hole swimming area = found $3.48 a ring an earring. Lots of fun, I love that place, we come from Houston to stay there every year. The people in the park said that at the end of the summer season Sept- Oct time they pull the metal plates on the damn near the falls and by the blue hole, that's when they have the most people come use their metal detectors when the water level goes down.