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mith, that's common in many places. often, a good app will either give a link to a recommended registrar (whether placed by the admin or the developer, makes no difference. users need to be given suggestions where to register)
code *could* be developed to make registration interact with the registrar from your site, i believe some provide scripts for that purpose. it just requires integration into phpcoin or whatever you use.
as for subdomains, by default they are searched for on whatever server the main domain exists on. so if a domain is registered, once it propagates, all subdomains will be detected at that same location, as long as they exist