A few weeks ago I ran a poll asking does the alt tag improve rankings in search engines. We have now received over a hundred responses from our readers, who I assume are mostly SEOs.
About 80% said, yes, using an alternative attribute on images does indeed help improve rankings. While about 20% said it does not.
Here is the breakdown:
:: Alt Tag improves rankings said 86 respondents or 79.63%
:: Alt Tag does NOT improve rankings said 22 respondents or 20.37%
With most these polls, sometimes people look too much into the question. I am surprised about 20% said no.
Forum discussion continued at WebmasterWorld.

Comments:
diarmuid ryan
12/21/2008 07:06 pm
i have not put it to the test, but i must! i guess it is good practice to always use alt but sometimes it just gets forgotton about
Scot
12/22/2008 04:48 pm
I can understand the possibility of it not helping the page overall. I disagree, I believe it does help, but I can understand it. However, it absolutely must help the image and image searches/results. And in the age of Universal Search Results your images need to be as optimized as possible so those ALT tags better be good.
Terry Van Horne
12/22/2008 07:00 pm
I agree it depends... for image search it is important... for link text it's useless... and I strongly doubt it is weighted for organic. That's the problem with general polls on SEO techniques. I bet if you excluded universal it would be the opposite result.
Nicos
12/30/2008 01:05 pm
alt-Tags are one of several "small screws" that can be adjusted while optimizing onpage! having keywords in your alt-tags makes seo-measures complete! Not to forget that alt-tags really help in getting your images into googleImages-searchresults, which may bring some more traffic together with organic serp traffic....