Say I want to develop a site with some CMS embedded in it – e.g., a WordPress blog and phpbb forum.
What’s the most convenient way to unify the log-in and sign-up process for my site?
Letting users have a different registration process for each part of the site would probably scare any sane user away.
EDIT
I prefer generic solutions, not necessarily for the aforementioned CMSs.
Both WordPress and phpBB3 have OpenID authentication plugins — you could use OpenID for both.