A rather interesting thread in the Google Webmaster Help forum addresses the role that alt tags (and stuffing those alt tags with keywords and phrases) may play in a sites' ranking.
Impressions and visits to the forum poster's site dropped dramatically after a site redesign. After reviewing the site, Google's John Mu pointed out that the alt tags being used by the Webmaster were, for many of the products, utilizing the full copy of the title as the alt-text for the product images on the page. A big no-no if ever there was one in SEO.
"Keeping in mind that the alt-text is meant to be an alternative text that could be shown in place of the image (and accordingly, it's indexed as part of the page), it could be confusing to see the exact same text over and over again," said Mu. "Search engines generally aren't impressed by seeing the same text that many times, so I'd simplify that a bit by perhaps using the full text for the main product image, but not reusing it for all of the smaller detail-images."
Using alt tags is definitely best practice but keyword stuffing (anywhere really) is not. Keep alt tags short, descriptive and relevant to the image they are being used for and you'll all but guarantee that alt tags help (and not hurt) your rankings.