1) In my first unit, which focused on korbanot, I essentially had the students learn the material that I used to teach, and the project asked them to take that material up a level by envisioning a 21st-century Beit HaMikdash. On the whole it worked well, insofar as the students did learn the material and did manage to find areas where technology could be integrated. However, the leap from material to project was fairly small.
For this new unit, I am planning on having the old curriculum material serve as merely a launching pad for a more comprehensive study of modern kashrut. The unit is based on Devarim 14, where the Torah presents a list of kosher and non-kosher animals. Our study in the past has included reading several articles about animals of questionable kashrut status, as well as comparing these verses with the similar list in Vayikra 11. While I plan on keeping all of that material in, my goal is to construct a learning experience that uses those sources as a way to teach certain principles that guide modern kashrut, and for the students to take advantage of the many resources available from kashrut organizations (OU, Kof-K, Star-K, and CRC, among others, have extensive reading materials online). While I have not yet finalized what the project itself will be, there will definitely be a greater gap between this unit this year and the same unit in past years than there was for the korbanot unit.
2) Taking a broader perspective, one of the beautiful things about a PBL unit is that it actually helps to transition students to a higher level of learning. When students are younger, their learning activities and experiences are wholly guided and directed by their teachers. As adults, we are in full control of what we learn and how we learn it. In between, there should hopefully be a gradual shift in the locus of control when it comes to learning - witness the existence of electives in high school and a college student's freedom to choose courses and concentrations. By introducing PBL in Middle School, I am easing students into the next level of their education - I am still controlling the overall curriculum, but students are given choices about what sources to learn and at what pace they proceed. They also have substantial latitude about what the final product should look like. And, of course, I am always around. Unlike at higher levels of education where a student often has to chase down a professor and sometimes has to go it alone in between meetings with his or her advisor, my PBL students work mainly in the context of our class, which means that they have full access to me to help guide them to the next level of their learning.
And, of course, that is what I like the best about PBL - it moves towards more "authentic" learning, where the learning is driven not by the curriculum of the teacher but by the curiosity of the student.