According to Chris Carella and Giff Constable, both of the Electric Sheep Company, have indicated today that the Electric Sheep Company is looking to contribute bug-fixes and some modifications back to the open source Second Life viewer.
While the Second Life viewer source code is mostly under the GNU Public License (version 2), however the viewer is actually dual-licensed, with proceeds from the commercial license being used to help fund further development.
There have been some objections to the Electric Sheep Company not releasing the code to their OnRez viewer, under the assumption that the source code was obtained under the GPLv2 license. Constable asserts that it was not, and that it was obtained under a commercial license from Linden Lab.
Nevertheless, Carella and Constable are looking into portions of the OnRez viewer which can effectively be contributed back to the open source viewer tree.
Additionally, while there is no Linux version of the OnRez viewer presently available, and no immediate plans, Constable acknowledges the growing demand for it and has not ruled out that one may be released in future.