The GNU GPL for phpBB requests that link stay there, and like most I intend to respect the license.
It's also respectful. The folk who put in so many hours a day in to making phpBB a reality deserve credit. So the link stays.
Technically, this is incorrect. The GPL really only covers redistribution of the software, not usage or modifications for your own use.
I can remove the banner, replace it with "Powered by Monkeys", etc. etc. as I see fit as long as I'm not redistributing my modified version and passing it off as my own creation. If the phpBB team tried to impose additional restrictions to the software (like forcing the use of a "Powered By" banner), phpBB would not longer be licensable under the GPL (which prohibits additional restrictions).
That being said, I still have the banner up on my forum. phpBB is a great thing and I think the developers should get all the credit they deserve.