Hey Bob -
I actually did see a sorority while I was a senior in HS, because when I came to visit my host was Greek and she took me over to see her house. So, if you know or meet someone on campus who is Greek, they would probably be willing to give you a tour. However, open houses don't really happen until you are actually a student here.
Another option is to check out the Greek organizations websites - these usually have pictures of the house, sometimes like a virtual tour kind of thing, and they will also give other information about the chapter that you might be curious about.
You can check out Butler's Greek chapters at http://www.butler.edu/greeklife/?pg=526
If you are just curious about Greek life, this might be a good thing to do, but try not to develop opinions about specific chapters before you come here - you'll get the most out of your experience at Butler if you come in with an open mind!