A lot has been written lately about Nokia buying the Symbian shares of Sony-Ericsson and others and creating the Symbian Foundation to release the OS as open source in the future. A lot of people become ecstatic when they hear 'Open Source' as it seems to be a synonym for success and the only way to go. However, there are different kinds of open source approaches and usage licenses so it is worth to consider what developers will be able to do with Open Symbian that they can't do today.
I think the big difference to Linux, which is also open source and has attracted many individuals and companies to start their own distribution, is that I think it is unlikely the same will happen with Open Symbian in the mobile space. In the PC world, the hardware is well standardized so people can easily modify the kernel and compile and run it on their machine. In the mobile world however, hardware is very proprietary so I think it is unlikely that the same will happen here, no matter how open the Symbian OS becomes. Therefore, an open Symbian is mainly interesting for hardware manufacturers as they will have easier access to the OS and can customize it more easily to their hardware. That's a long way from 'I don't like the current OS distribution on my mobile so I download a different one from the Internet and install it on my phone'. But maybe we are lucky and open sourcing the OS will allow application programmers to use the OS more effectively and extend it in ways not possbile today due to the lack of transparency.
For more thoughts on what the Symbian Foundation might or might not change in practice, head over to Michael Mace and AllAboutSymbian, they've got a great insights on their blogs from a lot of different angles.