A while back, I was faced with a challenge; i needed to cookie my users with information found in my vBulletin database. In this example, my users are already logged in, it was a matter of stamping the pligg cookie to the user so he/she can post/comment/rate.
Take a look at the script, it is a basic example of what to do.
basicly, i inserted a row into the pligg user table for every user that was cookied.
upon further review, i found it easier to stamp the users with the same password in the users table, thus i wouldn't have to pass a password in the url.
this code should get most of you advanced programmers going, if you have any questions, i will do my best.
check out the implementation:
http://www.zeropaid.com/
The code is still in development, so don't cry wolf!