@Jon Brown -
@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.

In a legal sense, that statement just isn't true. There is absolutely nothing legally binding to the GPL anyone creating a WordPress Theme, because a Theme is not inherently derivative. The only thing that would cause a Theme developer to "agree to the terms of using [WordPress]" would be to incorporate actual, copyrightable WordPress core code into the Theme. Other than that instance, the developer is in no way bound or encumbered by WordPress' GPL.

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.

Simply put: no. The courts have been quite clear: mere linking does not render a work derivative. See Sega, Sony, and Galoob.

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.

Again, this interpretation is at odds with precedent case law. Function calls are "methods of operation" and the means of interoperability, which the courts have clearly established are non-copyrightable content.

A theme is fundamentally interwoven with wordpress code throughout...

No, a Theme exists of its own files. Even when a Theme is aggregated with WordPress core by being placed in the wp-content/themes directory by the end user (not by the developer - a key point), no physical interweaving of WordPress core and WordPress Theme files takes place. A Theme does not physically modify any core WordPress code, and vice versa.

...witness Drew Blas’ analysis of Thesis a theme which Pearson regularly claimed was 99% his original work.

What I'm talking about excludes the Thesis issue - about which I cannot speak with any certainty, since I've not seen Thesis' source code. I'm only talking about Themes in general, under the assumption that core-code copy-pasting is not taking place.

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).

I would contend that merely extending WP_Widgets is not derivative. Modifying a default WordPress Widget would be derivative, because such modification would require incorporation of that Widget's code into the Theme. But merely extending WP_Widgets to add an original, new Widget is not derivative. In this case, the Theme developer has merely followed the functional specification for adding Widgets.