One of the reasons its so popular is because of the free useage of it.
I think if they really felt like they needed some money for it it would be much more logical to ask for donations, or use the phpBB site as like a gateway to promote other projects that are pay projects. Also they could always offer a fee just to download it but keep it all opensource and free still but then people would probably just find other places to download it from. Currently all the downloads come from sourceforge anyways so it doesn't use any of the dev teams resources for people to be downloading copies from them all the time.
If it ever did become no longer free I think other free ones would slowly start to prevale over it. vBulletin is a very very good forum, some features are better than phpBB's and some of phpBB's features are better than it but if they were the same price and I have to choose between the two I would probably choose vBulletin. Although phpBB 2.2 is going to be pretty darn wicked, vBulletin is also about to release a 3.0, they are at beta 4 last time I checked.
Anyway, like I said I think if they were to start charging, another free board would rise up to the level that phpBB has currently in the community. Probably openbb at openbb.org, it still has a lot of work to reach the level phpBB is at but it's an extremely promising and good board.
I really don't think there would be that much advantage to the phpBB to start charging for it and I really hope they do not.
Although I have never heard any info directly from the team themselves that they would. Some people just really enjoy making free software and contributing to the community. I tell ya if I was a rich millionaire I'd give the team a bunch of money just for doing such a great job and putting so much effort into it. Besides, I'm sure they get return in ways other than money. Also I'm sure "I am the lead developer for phpBB" looks damn good on a resume and is worth something in itself