This comes from the PayPal subscription info:
With Subscriptions Password Management,
PayPal can generate unique
usernames and passwords for your
subscribers, enabling them to access
"member only" content on your
site.
After a customer signs up for a
subscription, he will be given a unique
username and password on the PayPal
Successful Completion page. You
will then activate the username/password
pair on your website's server
through code you write.