Bootboy and everyone else, I deny using Bobby, since his reports are not 100% valid.
For example when using empty or null alt tags, e.g alt="" or alt=" " with non-meaning images, as blind or spacer gifs, you do not pass priority 1 of the W3C/WAI WCAG 1.0, even if you really do.
More:
http://www.w3.org/TR/html401/struct/....html#adef-alt
Try CynthiaSays. That is an accessibility validator I can trust.
For Mark-Up and more, see here:
http://www.webnauts.net/check.html