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The most comprehensive ALT tag resource I've seen so far is Use of ALT texts in IMGs [ppewww.ph.gla.ac.uk].
I couldn't agree more. You can usually get your keywords into a "legitimate" alt tag that won't have vision impaired people going nuts.
Also, Google displays alt content right there in your site description -- that can make for some awful looking listings if you over-stuff them with keywords.
ALT tags and persons with disabilities, screen readers for the blind make use of that tag, as do wireless (WAP) devices and web clippers. In the case of screen readers, the ALT tags are read aloud in lieu of the graphics by the machine doing the text to speech conversion.