Email-only subscription form with mailchimp module?
I haven't got a mailchimp enabled Drupal site in front of me, but from memory the simplest way to do it is to set the fields you don't want captured as optional in the Mailchimp interface (NOT the Drupal module) and then just hide the fields using CSS on your subscribe form. 

If you don't need to capture anything other than email for your mailing list, you can remove those fields entirely from your list on Mailchimp's site, and (again from memory) they won't show up in your Drupal form.