I think we are going to agree that nillable should be signified in the @param or @return as follows:
/**
 * Service for managing email contacts
 *
 * @service
 * @binding.soap
 * @types http://example.org/contacts contacts.xsd
 *
 */
class ContactService {

    /**
     * Retrieve contact details
     *
     * @param string|null $shortname The short name of the contact
     * @return http://example.org/contacts#contact|null The full
contact details
     */
    public function retrieve($shortname) {
    }

}

I have fixed this in the DUNLIN branch.

There was discussion about this on phpsoa - the sample below illustrated both the | null idea which is implemented in 1.2.2. and the notion of # to separate name and type which is not. So I am closing this defect. Please feel free to reopen if you find the problem is still there.