If you want a good insight on the issue of legal implications of creating derivative/non-derivative works with functionality that is present in existing implementations, I suggest that you follow the SCO vs. Linux lawsuit, the arguments around the issue are very relevant to your question.
For what it's worth, you could even try to revamp the old AT&T claim against BSD. Here is where you might want to start: http://en.wikipedia.org/wiki/USL_v._BSDi http://en.wikipedia.org/wiki/SCO_v._IBM_Linux_lawsuit P.S. If you don't know already: SCO is the company that now has more lawyers than it has hackers.
