Digital Web Magazine - Features: Accountability of Accessibility and Usability

Southwest's site committed the cardinal sin of Web accessibility: They did not use the ALT attribute for images that convey information. While ALT attributes aren't necessary for spacer images or purely ornamental images, ALT attributes are essential for all images that convey content. The text in an ALT attributes describes to users of non-graphical browsers (including screen readers) what information is displayed by the image. On the Southwest Home page, their company's logo didn't even have ALT text.
http://www.digital-web.com/features/feature_2002-11.shtml