I don't understand why just because something requires Wordpress to work it is a derivative work? Surely anything designed to work on any platform would be a derivative work by that argument? The GPL talks about containing the original Program, not requiring it.

Looking at the quote below from the Software Freedom Law Center, if you replace "PHP" with "VBA" and "Wordpress" with "Excel" it would imply that all Excel macros are derivative works of Excel and so only Microsoft can distribute them.

The following is taken from http://wordpress.org/news/2009/07/themes-are-gpl-too/, bits in [] are my addition:

The PHP[VBA] elements, taken together, are clearly derivative of WordPress[Excel] code. The template[macro] is loaded via the include()[whatever] function. Its contents are combined with the WordPress[Excel] code in memory to be processed by PHP[VBA] along with (and completely indistinguishable from) the rest of WordPress[Excel]. The PHP[VBA] code consists largely of calls to WordPress[Excel] functions and sparse, minimal logic to control which WordPress[Excel] functions are accessed and how many times they will be called. They are derivative of WordPress[Excel] because every part of them is determined by the content of the WordPress[Excel] functions they call. As works of authorship, they are designed only to be combined with WordPress[Excel] into a larger work.