Here are a couple of references from Google sources:
Read the full post + the comments below for reference to the keyword tag.
"Next we have two name values: keywords, which these days is mostly useless, ironically, and description, which is still somewhat useful."
As for the other search engines, the only other one I somewhat care about is Yahoo, and my sites do very well in their results without there being a meta-keyword tag on my pages.
If you spend a few minutes searching your favorite SE, you can find dozens of references to this topic