One of the common features of any site offering a self-provisioned service these days is a captcha (an acronym for "completely automated public Turing test to tell computers and humans apart"). Google's account sign-up page, for example, has one. The idea is to stop automated sign-up from people who would abuse the system for spam or some other nefarious purpose.
A friend of mine, Nathan Sandland, just launched a company offering a word verification product written in Java. Nathan is a great programmer and someone I'd love to work with again someday. I wish him luck in his new venture.
I think this would make a good Web service. The chaptcha image could be generated and named with a hash. The site requests the URL of an image, takes the user's input and sends it back, along with the image URL for verification. This could happen using SOAP or REST using a secure connection.