US Copyright law was modified a few years ago so that a notice is no longer required to secure copyright protection. Your creative work has copyright as soon as you publish it.
So phpBB is copyrighted software, whether or not the notice is displayed.
Here is a reference to back up my statement: http://www.copyright.gov/help/faq/faq-general.html
The following has been quoted from that page for convience.
When is my work protected?
Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.
Do I have to register with your office to be protected?
No. In general, registration is voluntary. Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work. See Circular 1, Copyright Basics, section “Copyright Registration.”
Why should I register my work if copyright protection is automatic?
Registration is recommended for a number of reasons. Many choose to register their works because they wish to have the facts of their copyright on the public record and have a certificate of registration. Registered works may be eligible for statutory damages and attorney's fees in successful litigation. Finally, if registration occurs within 5 years of publication, it is considered prima facie evidence in a court of law. See Circular 1, Copyright Basics, section “Copyright Registration” and Circular 38b, Highlights of Copyright Amendments Contained in the Uruguay Round Agreements Act (URAA), on non-U.S. works.
In addition this page explains copyright notices: http://www.copyright.gov/help/faq/faq-d ... tml#notice
What is a copyright notice? How do I put a copyright notice on my work?
A copyright notice is an identifier placed on copies of the work to inform the world of copyright ownership that generally consists of the symbol or word “copyright (or copr.),” the name of the copyright owner, and the year of first publication, e.g., ©2006 John Doe. While use of a copyright notice was once required as a condition of copyright protection, it is now optional. Use of the notice is the responsibility of the copyright owner and does not require advance permission from, or registration with, the Copyright Office. See Circular 3, Copyright Notice, for requirements for works published before March 1, 1989, and for more information on the form and position of the copyright notice.
So it may be considered morally wrong to remove the copyright displayed at the bottom of phpBB pages when they are executed, but is not illegal to do so.