There are no long distance charges for calls made to the 50 United States, Puerto Rico, U.S. Virgin Islands, Guam and Northern Mariana Islands from any location in the United States. Text Messages sent from the United States to international telephone numbers cost $0.25 each. Pictures messages sent from the United States to international numbers cost $0.50 each. Text messages sent to any location while roaming internationally cost $0.50 each while multimedia messages cost $1.30 each. This link shows countries to which one can send multimedia messages: International Text, Picture & Video Messaging | Wireless from AT&T, formerly Cingular
There is also a link on that page with the list of countries to which one can send text messages.
