Notice: See updated post: http://www.sayakbanerjee.com/2011/07/14/identity-for-kde-forum-update/
KDE Community Forum (forum.kde.org) now has an option that lets you log in using your KDE Identity (identity.kde.org) credentials. The feature isn’t fully finished yet, and has a partial implementation as of now.
To use your Identity credentials, you must have a forum account for now. You can go to the User control panel -> Profile -> Account settings to link your Identity username with your forum account. Once done, you can then log into the forum with your Identity credentials by checking “Log in using KDE Identity” option on the login page.
Soon, it will also have the option to link an account right from the login page and/or create a new user.