There’s been discussion some time back (September or so) about the use of BSD-licenced code (wireless drivers from OpenBSD) in the Linux kernel; specifically, that changes to the drivers were only released under the GPL and therefore unusable by OpenBSD (which won’t accept any licence more restrictive than the original Berkeley licence).
Most of the people appear to be missing the point. Of course, the Linux developers are perfectly entitled to do so—the OpenBSD licence doesn’t require anyone to release any modifications at all, and BSD code is used by, for example, Sun, Apple, and Microsoft for this very reason.
However, that doesn’t mean they should. Just because you don’t need to release your changes doesn’t mean you shouldn’t do so anyway; Apple especially have contributed a fair bit back to FreeBSD despite not being required to. And from free software developers, a refusal to release code under a licence that’s acceptable to the author is a little hypocritical.
It shouldn’t be a case of "do it because you have to". It should be, as I’ve mentioned_before, something you do because it’s the right thing to do. The Linux guys know it’s right, yet still they avoid doing it.