Quote by: Not a lawyer, but...
Isn't it a breach of the GPL contract that you've removed the "Powered by Geeklog" statement (nor replaced it with something similar)?
No. The GPL only has any effect over Geeklog if you give it to someone else. If you do, you have to pass on a copy of the GPL, the source code (irrelevant since php IS the source code), and relicense all of your changes under the GPL.
Now, the professional theme is licensed under the GPL, but a theme MAY be licensed under something like CC-attribution or CC-noncom. Implementing an API isn't creating a derivative work (assuming it's from scratch), so whatever a theme creator would like to license it under is their own deal - you just have to follow the terms they lay down.