Matt Cutts – Google and Meta Keywords
Does this mean you don’t need to include meta keywords? That depends…. and is totally up to you. There are other search engines that might look at it – how much weight does it carry? Not enough to spend sleepless nights making sure you include tons of words and repeating words just to make sure you have that keyword in there multiple times to *boost* chances of a better position in ranking. If it’s a matter studying and stressing over keywords for the meta keyword tag or spending the time on creating better content – you’d be better off doing the later.
The system’s search uses the product title and keywords fields. If you use the internal search box on your site and need additional words you couldn’t fit into your product title so customers can find your item then yes, add them to the keyword field.
Example: product title is Polka Dot Bow you might put the color in the meta keyword field so that’s also picked up.
I’ve seen the meta keyword tag with things like : personalized baby tee, personalized baby hats, personalized baby things, personalized baby boy things, personalized baby girl things, personalized baby clothes. – there isn’t a need to add ALL those “personalized” words, it’s not going to help, isn’t needed, and is just a wasted time, time you could have spent on better content or marketing.
So in short – use it to add any extra descriptive words of your item that you couldn’t fit into the item title and spend your extra time on your content and marketing 
More info on Google and the Meta Keywords tag
http://googlewebmastercentral.blogspot.com/2009/09/google-does-not-use-keywords-meta-tag.html
