This is being talked about in w3c-wai-pf... adding this bug to keep it on radar. After discussing with Marco a bit on channel, I'm not sure we want to do this, but let's continue the discussion here. We can always WONTFIX later.
Currently the implementation is as follows (excluding ARIA logic):
1. If an alt text is present, and it is "", screen readers should treat the image as presentational/decorative.
2. If the alt text is something other than "", the text should become the image accessible's accName.
3. If alt is not present, but title is, use that as the accessible name.
So if the accName is NULL, screen readers should try to do their own magic. Only if the accName is "" should the screen reader truly ignore the image. This was done before role="presentation" came about.
Making all images that have no alt attribute go presentational would not only break the title logic, but also prevent screen readers from applying their own logic to something that might still be important, but poorly marked up.
I would prefer it if HTML 5 included a new attribute such as "presentational" or something shorter that would clearly state that an image is presentational. We could still apply our magic then for alt="", but this attribute would be available also for tables (brrrr) and other elements which might be mis-used for presentational purposes.
Just my 2 Cents.
I agree Marco, closing invalid.