Probably me and one other person on the planet will ever see this bug, but I found it in RC1 and it has followed to RC2.
I run my server on a non-standard port. This is set and verified in the phpBB ACP.
When a new user signs up, the URL that they receive in the email includes :80, not the correct port for my site.

Try it for yourself:
http://www.nycnyne.net:999/forum

The activation URL you receive will receive is
http://www.nycnyne.net:80/forum/blah

First, this is just broken and, second, it is redundant.

A sample email follows:

>
>Your account is currently inactive. You cannot use it until you visit the
>following link:
>
>http://www.nycnyne.net:80/forum/ucp.php?mode=activate&u=74&k=3DV3U74ELJ
>
>Please do not forget your password as it has been encrypted in our
>database and we cannot retrieve it for you. However, should you forget your
>password you can request a new one which will be activated in the same way
>as this account.
>
>Thank you for registering.
>
>--
>Thanks, The Management

What is interesting is that if the sign up is done over https, no port is reported in the URL and the link works.