Just wanted to say I love the sticky-note idea. I'm going to use that at some point. I struggle sometimes with extended, somewhat self-paced lessons, as some kids lag behind and get off task. I think this might help those individuals and/or groups stay focused.
I have worked out the details. Well for now. Take a look at my PBL lesson plans
A few things I see as potential problems are engaging all students in each group, engaging all students in brainstorming sessions, and making connections to turn one groups' failure or success into your own groups' success.