There’s a bit of confusion among many web designers and seo specialists surrounding the use of the
alt attribute to provide alternative text for images and other non-textual elements.
In alt as a tooltip, Anne van Kesteren points to a document on the Channel9 Wiki site that contains feedback about Internet Explorer Programming Bugs. The alt attribute (yes, It’s alt attribute, not alt tag) and how it is displayed by Internet Explorer is discussed in the “Browser Behavior” section. Several people point out that Internet Explorer is wrong in displaying alternative text as a tooltip while also displaying the image.
They are all correct. Unfortunately, judging by the response from one of the IE developers it seems Internet Explorer will continue to display alternative text as a tooltip.