There are a lot of people who believe the meta tag for keywords no longer is useful in SEO. This is probably true as far as someone searching the internet for a particular item that matches one of your keywords.
If you have google ads, you probably should keep using the tag up in the head of your page.
I put together a couple of test pages both identical except for the file name. One is called pink.pony.pub.html, and the other is a number only.
I made sure that none of the elements from the pages were associated. For example: I used meta keywords associated with elephants, and the picture was named "eagle" but I used "wolf" for the attribute "alt".
I was amazed to find that the ads on the page with the number name were all related to either keywords, or "wolf". The page with the file name pink pony pub, alternated with "Gulf Shores, AL" (the location of the Pub) and elephants, with the link ad at the top more often showing ads for "wolf" related things.
Keeping in mind that there was absolutely no content on either of these pages, it was easy to pinpoint where adsense was getting their cue for ads related to the page.
I found that once I uploaded a page with content, the ads began changing with some of the words in the article. One word that was in tags seemed to "trump" the meta tag more often.
Knowing now, that both adsense and SEs look for the tagged words and phrases I tend to use them more strategically than before.
It's also safe to say that the words in the url are looked at when choosing relevant ads as well. In sites like hubpages and blogs where you can't choose meta keywords for each post or article, it's probably a good idea to incorporate keywords into the title of your article or post since that is going to be the url/link for it.