How to (not) use GPL Software
Derivative Works

Binary-only kernel modules
In-kernel proprietary code (binary kernel modules) are hard to claim GPL compliant
Case-by-case analysis required, as the level of integration into the GPL licensed kernel code depends on particular case
IBM is in the process of getting rid of all binary-only kernel modules.  There are exceptions, but they are very clear ones (such as a filesystem port to linux, where the filesystem code already existed under another OS)
There is no general acceptance or tolerance to binary-only kernel modules in the Linux (development) community.  Not even Linus himself has ever granted an exception for such modules!