Sun tests the OSS waters with Solaris:
Sun, which has never completely rebounded from the tech collapse in 2001, hopes the no-cost of Solaris 10 will not only attract customers but also expand the number of developers who write programs that work on computers running the operating system.
The result, Sun believes, will be renewed demand for its servers and services. The company also will charge subscription fees for Solaris support and service programs that are typically sought by the businesses and organizations that Sun targets.
Well, that depends on what they do with the license. In general, free works for open source software - but does not work for commercial code. Sun seems like they get this - check this from further down in the article:
Sun also has promised make the underlying code of Solaris available under an open-source license, though the details have not been released. With access to the code, Solaris users will be able to take advantage of its features when developing their own software and systems.
So it's an open question to some extent. On the other hand, how likely is it for Sun to attract the kind of community that Linux has? Linux has a lot more time and momentum behind it. Time will tell.