Depends what you mean with "fork". The usage used to mean a big and ominous circumstance, like Emacs vs XEmacs or gcc vs egcs. Nowadays people use it to mean any deviation from a project, no matter how tiny, or even simply a server-side git clone, as popularised by github.
Nobody is obligated to accept any patches, no matter their provenance, no matter if they were paid for. My two parties above in a mutually beneficial agreement could decide to keep the patches between themselves if upstream rejects them. I suppose that could be called a fork. I don't think every such occasion should warrant a new name like under the traditional definition of "fork", though.
