I following one of the hundreds of links in my course and found Framing Essential Questions. It's an older article about problem/project-based learning, with some excellent examples and a list of characteristics of questions that spark real learning. My favourite: "Essential questions spark our curiosity and sense of wonder. They derive from some deep wish to understand some thing which matters to us."
It also includes a nice reality check to chase the idealism: "It would be best if students could learn to frame their own essential questions, but in most cases they will require several experiences with teacher generated questions before they can shed years of practice with trivial information-gathering questions."
I thought that really summed up one of my recent arguments -- the problem with progressive approaches to learning (constructivism, project-based learning, authentic assessment) isn't necessarily bad instruction/facilitation (although this may occasionally be the problem) or flaws in the learning models themselves...it's the fact that they're often tried on people of all ages who are used to be spoonfed meaningless hoop-jumping exercises, and the new methods require them to think, which is quite uncomfortable if you've never had to before.
As an extension of essential questions, Diane introduced us to the concept of I-Search assignments, which are research projects that require the student to study something they're actually interested in (what a radical concept), find compelling questions to answer, document the searching process, and create a paper and separate project as artifacts of the activity. Solid stuff, and not rocket science.