Response for Registering on Wordpress Site through iPhone
If you have access to the site, which I guess you do, you should be able to write a small plugin that let's you perform the registration by posting data to an URL specified by your plugin. This would be fairly simple, just hook up a function to the init action and check for the $_POST variable for any input.
Then simply use username_exists to check for existing users and wp_create_user to perform the registration. These functions will give return values that you in turn can send as a JSON reponse (or whatever is appropriate) back to you application.
In fact, my experience with XML-RPC is that it's somewhat limited, and not really up to date with the rest of WordPress, so I often make these little mini API's to handle situations like this. All that might have changed in the latest releases, however.