The conjecture is pretty simple. You work with an open source project because (a) you have to because that's your job or (b) because you want to. Symbian at the moment has a rather huge amount in the first category, while there are significantly less in the second category. For a truly successful open source project, they need to be in balance.
"The problem with Symbian is that very, very few people touch it for fun. So I believe that while we can open source it, it is going to be very difficult to get people participate out of their own free will, unless we are prepared to make very serious refactorings to the entire system.
"Still, it is going to keep Symbian in the game a little longer. While many people dislike it (just go to any Finnish IT newspaper discussion board to verify this), the fact is that it does carry a significant amount of gold home every day. So we must be doing something right, and my bank account would very much like to us keep doing that."
More at Nokia Conversations.