This plugin by Joshua Bettigole adds the capability to create newsletter opt-in forms with Contact Form 7. Contacts are automatically subscribed to the list determined in the form configuration.
Features include
Sends email and optionally a name.
Can either use “subscribe” checkbox or submit all email addresses.
Submit to any list on Campaign Monitor.
Works silently. Does not alert subscriber to errors or duplicates, and does not send a welcome email.
Plugin homepage with config info: http://www.bettigole.us/published-work/wordpress-contributions/campaign-monitor-addon-for-contact-form-7/