You can’t share cookies across domains that are as different as that
The domains need to have some common part like:
blog.example.com
forum.example.com
Second level domains must be the same, as sam said. (First level being .com, .net, .org etc). It’s for security… imagine the havoc if you had to buy every extension for your domain so that others couldn’t, and read your users’ cookie info.
So far, I experimented and if you comment out [$bb->wp_home] on config.php, it works. Enter the value of $bb->wp_table_prefix and $bb->wp_siteurl, and user table is being shared by bbPress and WP. Like here, I could have a WP user on WordPress.com and my user name will be valid for bbPress.org.
@intellivisio, I don’t know about the cookie part. I still have to investigate. My main intent was to be able to have the user input info only once so that they could be authenticated on both bbpress site and WP site.
@sambauers, bbPress did complain on install about cookies, but after I commented out $bb->wp_home, it continued with the install. I created a user once on one domain and it shared the user on both domains.
Having user names being the same on two domains (very possible) is different than having two domains share a cookie (impossible due to the security implications I mentioned above).