hi dana,
i have already done that too. you will have to write an extenstion that modifies fn_cookie_login_fetch_cookie hook.
You will have to supply fake cookie information so that punbb thinks that user is already logged in.
I have instlled this plugin in my website at http://zixini.com/. after registering, visit the forum and you will see that you are automatically logged in. no need to login again in the forum.
and please dont say anything bad about the site as i am the only one whos writing the code in my spare time, and i already know that it looks pretty bad.. so please dont tell me that too
if anyone needs the code, i will gladly post the code too.