Hmmmmm... 508 was in many ways kinda nanny state stuff but well intentioned. I would agree that it is better to make versions for specific disabilities rather than try to cater for every eventuality in one place. That is a sure fire recipie for mediocrity.
On a point of accuracy I would like to point out that you should infact be using the title part of the html img tag for this and NOT the alt tag which was intended for something else.
It's only because the early versions of IE displayd the alt tag and the image (it's an old unfixed bug) that it's so widely used. The alt tag actually should only be displayed when the image isn't, that is still it's intended use in WC3 standards.
For fun make a simple html page add a picture with the alt tag filled in and view it with IE the tag text will pop up when you hover over the window, now add a title tag as well that's different to the alt and you'll see that new text now.
Use the right tags for the right use and it will work across browsers people, that's why there are standards like WC3.