@Chris Bennett - you make a point, and I I've rethought it some, but I was NOT trying to equate dependent with derivative. I agree they are different cases. I spoke to strongly to suggest calling a single function made always made a work derivative. Here's the thing though, when you choose to make a function call of GPL code you agree to the terms of using that body of work, not just that function. I'm not saying you couldn't duplicate that function into your own code and claim fair use of it, just that calling it directly from a GPL body of work means you are agreeing to the terms of that whole work.

Further while I agree there is a difference between dependent and derivative, I still feel all themes are derivative, however some plug-ins could certainly claim to be dependent without being derivative however. A dependent work would be original from the start and could make a few calls to another work but not make wholesale use of the framework defined in the original GPL work. A theme is fundamentally interwoven with wordpress code throughout, witness Drew Blas' analysis of Thesis a theme which Pearson regularly claimed was 99% his original work. Extending the discussion, a plugin that modified WP's sidebar widgets for example I would argue is likely derivative, whereas something like pretty link or something that added 100% new functionality might only be dependent and not need to carry the GPL (even if it utilized some fundamental WP functions).