I'm not sure about european law, but here's the deal with american law:
Patent typically falls under civil suits, and in a civil suit if you can get 12 people (the jury) to agree with you, you win.
I could technically sue a friend for being a jerk, and if I could build a strong enough case to where the jury believes me I would win (scary, stupid, but true).
TYPICALLY, case law shows that patents are upheld except in situations such as what is being referred to as "prior art" here - I don't think that's the term in the states but it's something along those lines. However, given that microsoft, apple and dozens of other software publishers have been using elements like tabs for decades the odds of successful litigation (at least here in the states) would be pretty slim.
IF EU law is similar to american law, the smartest thing for phpbb (the group of people, not the product) would be to form a corporation (call it phpbb corporation) and make phpbb an asset of the corporation - in the states you would have to sue the party responsible for breaking the patent (ie. phpbb corporation). With a corporation the individual board members can't be sued for personal assets, so the EVEN IF phpbb lost a patent case, the MOST a company could win is the complete assets of the phpbb corporation - since y'all don't accept donations, my understanding is that number would be $0 and the ownership of an opensource piece of software that due to the license COULDN'T be converted to a non-opensource project, at least the code up to the point when the license was transferred.
However - that all presupposes that EU law is similar enough that the american system would translate. I'm not sure that it does.
As a general rule (but often enough broken, so PLEASE don't send me exceptions, I could probably list more than you could
) laws of common sense generally apply with this kind of litigation. That's not something you can count on, but it's USUALLY true.