[Atsutane]

Nothing secret. My forum is on subdomain while my blog is on main domain.

All i need to do is define define('COOKIE_DOMAIN', '.atsutane.net'); and define('COOKIEPATH', '/'); inside wp-config.php

And if you are using Bbpress 1.0 RC1 with WP 2.7.1, you need to defind this define('WP_AUTH_COOKIE_VERSION', 1); inside bb-config.php.

Another thing is, i let bbpress handle all registration and log in/log out process.

Hope this will help you