One current method of fighting robots that sign themselves or other people up for mailing lists, contests, etc. is to have a random alphanumeric string presented in a way that makes it difficult to OCR the string. What happens when OCR tech improves?
(Note, I believe that humna-grade OCR is considered to be an AI-complete problem, along with natural language recognition (though great strides are being made there in number recognition. Also see the language recognizer in Unreal Tournament 2004)