encyclo - 12:31 pm on Jul 29, 2008 (gmt 0)

images that are presentational in nature should have empty descriptions (alt="") so that it is not read out by aural narration devices

Quite right - and you could take it a step further: presentational images should not be in the page markup at all, they should be included as background images via CSS whenever possible.

As for the SEO aspect, I don't buy it - alternative text is not a substitute for on-page visible text, unless you're trying the largely ineffectual keyword-stuffing technique, or if the image is used as a link - in which case, the alt text should be descriptive in the same way as any other text anchor. Inserting presentational images as background makes the page lighter and more focussed for the search engines.

So there's no real debate - apart from the question as to whether the images are really presentational or not. If they are in any way descriptive or offer supplementary information not contained in the surrounding text, then you must provide adequate alternative information.