This is taken from 'Wireless Data Service Terms and Conditions'. I have highlighted some important areas in Bold.
II. ADDITIONAL TERMS FOR SPECIFIC WIRELESS DATA SERVICES
TEXT, INSTANT MESSAGING AND PICTURE/VIDEO MESSAGING
Messages are limited to 160 characters per message. Premium text and picture/video messages are charged at their stated rates. Standard rates apply to all incoming messages when in the U.S. Different, non-standard per message charges apply to international messages sent from the U.S. Text, Instant, Picture, and Video messages are charged when sent or received, whether read or unread, solicited or unsolicited. AT&T does not guarantee delivery of messages. Text, Instant, Picture, and Video messages, including downloaded content, not delivered within 7 days will be deleted. AT&T reserves the right to change this delivery period as needed without notification. You are charged for each part of messages that are delivered to you in multiple parts. Picture/Video Messaging, data plan, and Text Messaging may need to be provisioned on an account in order to use Picture/Video Messaging. Some elements of Picture/Video messages may not be accessible, viewable, or heard due to limitations on certain wireless phones, PCs, or e-mail. AT&T reserves the right to change the Picture/Video message size limit at any time without notification. Picture/Video Messaging pricing is for domestic messages only. When a single message is sent to multiple recipients, the sender is charged for one message for each recipient and each recipient is charged for the message received. Text message notifications may be sent to non-Picture/Video Messaging subscribers if they subscribe to Text Messaging.