This seems to be one of those things that people are evenly split over, for every module that prompts there is one that uses environment variables. For myself I would probably sit on the fence and check the environment variables and only if they aren't set then prompt. I would also probably ask if they wanted to do the tests that require a working network at all if the environment variables aren't set: there might be a variety of local reasons why they can't access an FTP server at the time they are installing the module. Also bear in mind that it isn't just CPAN testers who might want to be doing an automated install.

/J\