In case you are wondering, the callback is needed for:
a) Auto approval into different communities
b) Sending a customized e-mail to the approved member
c) Triggering code in a custom application
This callback could eventually be put into good use e.g. for dotlrn-ecommerce, triggering the payment processing once the user is approved to the community. Or, if you are a university with a credit point system where the students have x amount of credit points which they can use up per semester, deduct the number of points from their allowance.
