I can only make a wild guess based on your short description since I can't enter the site due to lack of newest flash player. The WWW namespace is filled with modules provide interfaces to various web services, including the very tool WWW::Mechanize. So, IMO, this is enough justification to put your module under that namespace. Since the + part seems to be very attached to the Nike part as a web service, combining the parts is more appropriate so NikePlus is what comes to mind.

However, it might be better if you can narrow a category under WWW itself. Beside this can avoid cluttering the namespace further, it will give an instant hint about what can be done with Nike+, especially for people who have no idea what Nike+ is. Examples are WWW::Search or WWW::Scraper. Similiarly, WWW::Blog::WordPress would be more appropriate instead of WWW::WordPress.

You can make new category to put under WWW if no existing ones matching your modules to form WWW::NewCategory::NikePlus. Or, if you think the service can be extented in the future with distinct set of features, feel free to just use WWW::NikePlus. The precedence for this are WWW::Google and WWW:Yahoo.

The very last, I think you still need to talk to modules@cpan.org before making your final decision. Good luck, puploki :-)