The currently published specs for HTML 5 at whatwg.org [whatwg.org] no longer require the alt attribute for an img element -- and the longdesc attribute is nowhere to be seen in the current draft.
Accessibility folks are debating these changes, but I agree with Lachlan Hunt:
Making alt technically optional doesn’t stand in the way of accessibility requirements, nor greatly impact upon accessibility evangelism. It just acknowledges the reality of the situation in the hope of reducing the prevalence of poor quality, automatically generated alt text.
"It just acknowledges the reality of the situation in the hope of reducing the prevalence of poor quality, automatically generated alt text."
I was under the impression that one of the driving forces behind this move was sites that flickr that tend toward having large numbers of blank or automated alt tags where there are millions of web2.0 users who don't understand the importance of a good text description.
I think the better solution if this is the case is to maintain the alt tag and simply encourage site owners to make image descriptions or (at minimum)tags necessary for uploading images through social network web interfaces. This would have the added benefit of having visual data that is directly referenced and described by the person who is uploading it.
Here is a Zen question for site owners - "If you don't want to tell me what it is, why are you putting it on my site?"