You can use my plugin called MailChimp for WordPress to do this.
When installed, you can select one or multiple lists in the plugin settings and use the following shortcode inside your Contact Form 7 templates.
[mc4wp_checkbox "Your label text"]
It will render a checkbox and when checked, subscribe the given email address to the MailChimp lists you selected.
PS. I realize this question is over 2 years old. Just answering in case anyone else stumbles across it. Some answers had been given but I am pretty sure this is by far the easiest way to do it.