Danny Sullivan did a great, comprehensive examination of current status of the Meta Keywords tag , and his testing showed that both Ask and Yahoo will still use content in that tag as a relevancy signal. Both Google and Microsoft Live do not. His clear outline of the history, common questions, and contemporary testing of the factor were really helpful.
However, I think there’s still a case where Google may be using the Meta Keywords tag…
If you’re unfamiliar, the Meta Keywords tag is placed within the <HEAD> of webpage code, and it was originally intended to inform software agents as to what the subject matter of the document was. For instance, a page about football could have a meta keywords tag like this:
<META NAME=”keywords” CONTENT=”football, sports, NFL”>
Just as Danny outlined, this hidden keyword content with webpages was rapidly abused by many webmasters, as they added in terms unrelated to their page’s content, and stuffed repetitions of keywords into the tag in hopes of ranking higher.
Many search engines stopped using it altogether for keyword relevancy assessment and ranking, and even those search engines using it today may have considerable checks and balances in place to try to detect attempts to abuse through the tag.
I actually think that Google may still be using it as a factor for detecting spammish pages. As Danny’s testing shows, they’re not using it at all for relevancy nor ranking on content in the tag. But, it could still be useful to them as an indicator of sites which are attempting to use black-hat methods to manipulate search rankings.
It’s seemed very clear that Google is computing a quality score  in association with web-pages, and meta tags that are stuffed full of words that are highly unrelated to the content on their respective pages would be a prime indicator that a site may be attempting to use black-hat methods. It could be used as a strike against a page, but not as a factor that would help a page.
Danny reported that Google engineers have confused people to some degree by inconsistently communicating about the tags. Danny says that the engineers will answer the question of whether they “use” or “read” the keywords metatag literally, by saying that Google “reads” the tag, which leads people to think that Google uses the tag contents. Danny’s interpretation is that engineers like Evan Roseman are only stating that Google takes and caches the entire page code (which would require that their Googlebot spider “read” the page), but that they don’t use that part of the page for ranking/relevancy – they ignore it as a direct ranking factor. True, but another interpretation could be that Google is using it only as an indirect, negative factor as I’m suggesting. Google doesn’t want to expose all of their criteria for building quality scores, and that could explain the slight inconsistency in messaging over this topic.
So, Danny’s article shouldn’t be taken as an excuse to begin resurrecting meta keyword tags in a big way, with all the traditional abuses which destroyed the intended function to begin with. As he points out, it could be useful to use for misspellings, but use of it should be extremely conservative or not at all. I vote for avoiding it entirely.