ALT tags are only indexed by a couple of the search engines (Infoseek and Lycos I believe, but don't quote me).
The reason I don't recommend stuffing your ALT tags with keywords is due to one of my basic principles: never sacrifice the look or content of your site in favour of keywords. What is the purpose of ALT tags? It's for the convenience of users browsing your site with their graphics switched off, right? How appealing will it be for somebody to see spam all over your page? Hmmmmm...?