Last year the Symbian foundation ceased to be and Symbian stopped being an open source and was bought in house for to Nokia. Now Nokia have made it available again, but not quite in the same open source was as it was previously. People wanting to make contributions now need become approved collaborators and have their contributions approved by Nokia. Collaborators will need to get an alternative license called the Nokia Symbian License
You can view the Symbian blog and announcements at :
One thought I have on this is that perhaps as a open source project, open source development contributions slowed down the speed of development in the core platform and were not always helpful. This means that Symbian development should happen more quickly with collaborations that are more focussed on useful additions to the codebase and less resources trying to integrate contributions of questionable value to most consumers.