When you make a page about something, do you target both single and plural versions of a keyword, or only the one with the most searches? Overture keyword data combines the too, but i often see different serps in google or yahoo when searching for single and plural of a keyword.
Definitly plural keywords, unless the single keyword has result competition under 1 million, then I will also targetting for that single keyword (it's a rare case or perhaps I don't know how to find that keyword)
Some of the top search engines consider the plural keyword to be different from the singular keyword so I think you are better off optimizing your site for both the plural and singular versions of your keywords, assuming that both versions are popular.
I also target plural keywords and in almost all cases I rank high for their singular keyword also. Once you already start ranking high, you can play around and further optimize / refine to improve the results on SERPs.
Your best bet is simply writing naturally. This way you will not only use singular and plurals, but also derivatives (stemmings) as well. You can also use plurals in the URL and in the anchor text to the page, btw.
For a clients website, I optimized the pages for plural keywords, but after a few days, the site was ranking on first page on Google for both the plural and singular keywords. The keywords/phrases I was targetting had a high competition (around 20 million). That was good
I target plural keywords, when I use AdWords plural gives me more trafic than single word (and it usualy more difficult:/ )