actually, have you considered writing an authentication provider for salesforce - I know they use oauth2.0, so you could authenticate and read back the users social graph and use that to populate whatever fields you like. You could even add role membership (if you look at the active directory authenticate provider you'll notice it synchronises active directory roles with DNN roles based on the names matching). It's somewhat merging registration, authorization and authentication into an authentication provider but it's a technique I've seen used elsewhere (e.g. active directory, shibboleth etc.)

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Actually we use a mixed authentication: the user/password resides on DNN but... the role information are on Salesforce, managed by the Sales team. During the login process we check the Role on Salesforce and we synchronize it with the DNN role.

At the end of our registration process the user information will be stored on Salesforce, so a custom Authentication provider should be a good solution. With this solution we need to store the user pwd as custom field on the Salesforce contact.

The only drawback is that without a connection to SF no user will login on our Web site... 

Thank you for your answer, I'll do further investigations.