I'd like to belive in that too, but it works diffrent way on my test site.
It keeps data from a previous/last user in checkout form and basket widget. No matter if You are registered or not.
Maybe there is a way to add custom fields in user registration form, like address (is this what CIM is for?) which are visible in User Profile, and then synchronize wp e-commerce with those data, so when user logs in, he gets checkout form filled with his data. But as I was looking for this solution on forum I found several threads where people were saying, that WPEC has not such a functionality.
Maybe You got any tips?