There were many misunderstandings around this term in the past, but, as you already said - and your copyright notice - in the source. HTML template is also our source.
Actually, in this case GPL means that you can not publish or re-distribute our product ONLY with your copyright (for example, at the bottom of each page - copyright only of your site, actually, this is prohibited). You can not tell people that you have wrote miniBB, you can tell that you have modified, but in this case, you also need to tell - you have modified WHAT. You can build your own software based on miniBB - but in this case, you also need to tell - build on WHAT.
With the next version 2, we are planning distribute miniBB only under its own license (probably, BSD-like license), no more GPL.