Adding a required registration field

Hi.
I’m using WordPress and BBpress to build a blogging platform for my organization. However there are a few modifications we need to make. First and foremost we need to require the user to supply their zipcode when they register, and insert it into the database. I was able to add a new required field with no trouble at all but I’m totally lost trying to follow the flow control through WordPress and BBpress to find out where it takes the information from the fields. Can anyone tell me what code needs to be changed to do this? I have the zipcode being passed to register.php in BBpress but after that I get hopeless lost trying to figure out what the system is doing. Can anyone give me a quick pointer in where to find the code I need to access to drop the new field into the DB? I’m trying to use as much of WordPress’s and BBpress’s own code as possible.
Thanks a lot