From the Symbian blog:
Symbian Is Open
As of now, the Symbian platform is completely open source. And it is Symbian^3, the latest version of the platform, which will be soon be feature complete.
Open sourcing a market-leading product in a dynamic, growing business sector is unprecedented. Over 330 million Symbian devices have been shipped worldwide, and it is likely that a further 100 million will ship in 2010 with more than 200 million expected to ship annually from 2011 onwards.
Now the platform is free for anyone to use and to contribute to. It is not only a sophisticated software platform, It is also the focal point of a community. And a lot of the foundation’s effort going forward will be to ensure the community grows and is supported in bringing great innovations to the platform and future devices.
Over the next five days we’re going to bring you a raft of information on Symbian and the platform migration. So stay tuned for the full story – background, current status, technical details, the vision for 2010 and how to join the community.
It was my understanding that Symbian already was open source, but apparently today was the official announcement. Interesting footnote: Symbian is based on EPOC. Back in the 1980's, EPOC was originally developed by Psion for their line of handheld devices. Psion took EPOC OS through (I believe) 4 generations of devices before selling it in 1998.