Introduction to Engineering Design, the course I co-teach with Ann Saterbak was recently mentioned in a published discussion about instructional design. The discussion was between Josh Eyler, Director of Rice’s Center for Teaching Excellence and Sean Michael Morris. Among other topics, they touch on the importance of discovery for learning.
Like you, I firmly believe that curiosity and discovery are the foundation for learning. Students need to wonder; to be puzzled; to try, fail, try, fail, and try again in order for them to build knowledge and make meaning. Much of the discussion here, then, probably comes down to the issue of terminology. For me, PBL and inquiry-based learning are large umbrellas under which sit a variety of strategies. Certainly, as you say, there are some courses where activities might be rooted in discovery, but the discoveries are either already predetermined by the instructor or there is so much structure that it inhibits the process itself.
On the other hand, in just a few minutes I am heading over to take part in a pitch session for our ENGI 120 course. ENGI 120 is a Freshman Design course in the School of Engineering. At the beginning of each semester, the students listen to pitches from folks at Rice and the larger Houston community. They then vote on the projects they would most like to work on and spend at least a semester trying to design a solution. In the past, they have developed mechanical limbs, housing for birds at the Houston Zoo, and a device (featuring a regular old mousetrap!) designed to treat dehydration in children in African countries. Students work in teams, and they receive guidance from Ann Saterbak and Matthew Wettergreen, two of our faculty. The answers are unknown, and there is no guarantee that the projects will be successful. Students are simply given the freedom to explore and create with the goal of making real change. This, to me, is PBL at its finest, but I am also aware that many courses look far different from this.