Google has made it official that the keyword meta tag is not used in their search engine ranking algorithm.
Here is a short video by Matt Cutts on September 21, 2009 making it official.
In my opinion, the keyword meta tag should still be used to help track the words you are trying to promote on that page. It is a valuable reminder of what the goals are for the page. In addition, other search engines and spiders review the keyword tag.
My main concern for Google comes from people who fill the keyword meta tag with 50 keywords and keep them the same on every page of the website. This certainly does not help, and more than likely raises a flag to search engines indicating a potential spam site. So, if you use this field be sure to use it as designed, for a few keyword relevant to that exact page.
Best Practice for Keyword Meta Tag: Leave it blank, or make it unique to that page only including the 2-5 phrases used to optimize that page.