Even so, as the material noted by Graham states, the copyright has to remain unambiguous. A "merged" copyright is ambiguous in the sense that it's unclear who the original authors of software people are looking at are. In any event, distinct copyrights would seem to protect the interests of everyone.
Floriano, are you speaking about the content in your forum? If so, the content is generally given copyright protection to the author (your users). In the US, it's automatic, although labelling copyrights with a copyright notice and symbol help.
If it's a question regarding who wrote phpbb software, and how that work should be labeled, I think that question is pretty siimple: the phpbb authors wrote it, and you should give that credit to them.
The appropriate phrase is "Powered by phpBB", correct?
As noted elsewhere (including a ZDnet item out today), this phrase is a target of google-based searches designed to identlfy potentially vulnerable sites.
Would displaying a graphic image having this phrase satisfy the license? That would thwart the google searches, and all skin developers should be encouraged to include an appropriate graphic and a simple means of selecing it over the text copyright.