Ahh.. nothing like a good, theory-centric, hype-free approach to SEO. There's an extremely sensible and (fairly) lightweight Meta Keywords plugin rattling around over at data dump.
It seemed to me that a basic meta keywords tag should be trivial to include. According to Google’s
word of mouth advice, meta keyword tags (apart from being mainly unimportant) shouldn’t contain more than six or seven keyphrases (not keywords, keyphrases - more than one word can go there) separated by commas.
Sounds simple, right? This plugin reads your document’s content and title, strips out crap (formatting, useless words such as “the”, invisible characters) and then performs n-gram analysis to get an idea of term frequency. That’s a fancy way of saying that it finds the most oft-used words and phrases in your post. The top six (or seven, or even twenty - you can edit a constant at the top of the plugin to alter this) phrases are then plonked out into a nice meta keywords tag in your HTML header. Easy as pie!
Go and have a look for yourself, and leech the thing.
Meta Keywords plugin for WordPress