WordPress Text Message is a plugin by tprattmedia and hallsofmontezuma that alows people to subscribe to SMS text message updates of your web site. Manually send out texts, or every time a post is published.
There is a sign up sidebar widget.
Features
sidebar widget allowing people to signup for SMS text updates
ability to add subscribers manually from plugin settings page
support for all the major wireless carriers
manually send updates to all subscribers *option to update all subscribers each time a new post is published *ability to manage and delete subscribers
Download and info; http://wordpress.org/extend/plugins/wordpress-text-message/