That's a really good question. As I understand it, under the GPL any derivitive work must release the source code that was used. Isn't Limewire Pro basically a derivitive work of the free version?

Then again, Limewire Pro may contain some code altered enough to no longer require coverage under the GPL from the free version. In which case they wouldn't be required to release those parts of the code.

[P.S.]
As I understand it a link to download the source isn't required anywhere, but the source must be available by request (example: asking for via email.)