I've changed things a bit now.

I have a button ("Join") that does the following:

1. User clicks "Join" which performs a Response.Redirect to the portal's "Register Account" url.  You can find the URL for your site by using the default "Register" link and copying the URL from the address bar once you're there.
2. The "Register" script accepts a "ReturnURL" variable which is where the user is sent after they have "Registered".  I set this "ReturnURL" to point to admin/sales/paypalsubscription.aspx?roleID=XXX   where XXX is the roleID of the role that you want the user to be subscribed to.
3. Once the user completes the "Register" form, their basic user account is created and they are redirected to PayPal by calling paypalsubscription.aspx.
4. After the user completes the PayPal payment, and confirmation is worked out via paypalsubscription.aspx, the user is sent back to your site (the URL they are sent to is determined in your Portal -> Payment Settings area) and they should be included in the appropriate Role that you wanted them to be subscribed to.

It's all really simple once I got into it.  I actually had written a whole module that was calling the core sub's to create the user account, etc. until I realized I could just redirect to the appropriate URL's and let the DNN core handle it easily.

I like having a separate button that I can place on the homepage for the user to Register -> Subscribe -> go to the "members" section, without having to Register -> go to Account Profile -> Manage Services -> Subscribe.