I enjoyed this a lot. I don't agree with the premise that being in a group in the office is either necessary or efficient when it comes to idea generation. My own belief (and I would love to see a study on this) is that--more often--the steady stream of non-work-related BS going on around you means you lose focus and become less productive. We use teams a lot...with open discussion groups as you suggested, impromptu meetings when someone does come up with an idea, and small group ad-hoc "water-cooler" meetings among colleagues working on problems. 

What might be nice in these ad-hoc brainstorming sessions is to set up roles and rules for these sessions based on something like DeBono's five hats...Maybe color-code comments, or add a circle with black or blue or one of the other colors, and try to (as a group) make sure that all five colors are represented.