Using Sitecore and Web Forms for Marketers, how would one go about outputting the result from a webservice on the "success" page or similar.
How could we output this response from the WebService?
It depends. If you have a generic success message per form, you can set the success page url to a page and load appropriate sublayout to pull from your service.
If the success message varies based on the information the user entered, you could create a custom submit action and take care of routing to the right success url with url params to support the logic.