Here is the deal...
They have been letting limited numbers of random guests onto the ride to give the cast members practice loading and unloading people. The official previews themselves though are only for Cast Members, AP's, or DVC.
They may or may not allow other guests to ride at unanounced times though to basically give the CM's at Soarin' something to do. They did this when I was in FL for spring break, which was to make sure the CM's were good at handling guests, making sure the lines were as they needed them, and practice running the ride with people on it.
There really isn't a way of knowing if or when they will do this unless you actually walk over there and look. I have a friend who works on Soarin', and he said that he doesn't even know if they are going to let people on for what they call the "sneak peek" until he gets into work. Like I said, it's completely unanounced - but if there is a line over by the land & figment - Jump in!