Like many, I stopped using the Keyword Meta tag but here in a Yahoo Webmaster Page it recommends using it for their Bing-ed up SERPs. Does Yahoo know something about Bing that we don't know?
http://help.yahoo.com/kb/search/higher-rank-website-sln2216.html [help.yahoo.com]
Use a "Keyword" meta-tag to list keywords for each page of your website. Use distinct lists of keywords that relate to each specific page on the site. Do not use one broad set of keywords for the entire site.