Here's some real data on plurals:
A site we run was optimised for a singular version of a word a few years ago. Initially it just ranked for the singular.
However it has recently been performing equally well for the plural, even though the plural rarely appears on the site.
The best illustration of this is comparing the single word search for the singular and plural on Google (the word is the generic for an industry sector):
singular = #15 from 9,000,000+
plural = #15 from 10,000,000+
Google (by far the biggest source of SE traffic in the UK) easily deals with plurals and words that have the same base meaning (look up stemming or lemmatization). However, this isn't always the case with Yahoo and MSN.
You can see this behaviour by trying a few searches, try Googling 'walks' or 'walking' and look at the bold words, it's not limited to the exact word.
Walk => Walks => Walking
Also sometimes it goes as far as:
Magic => Magical => Magician
But where does this leave you when optimising?
My advice would be to try and use current stats from PPC or any other source to analyse the most popualar searches, are the singular or plural? Use the words that people are going to use in their searches.
e.g. if someone is looking for a lawyer, will they put in lawyer or lawyers? The answer to this often changes on whether they specify the service required:
defence lawyer in boston
lawyers in boston
I hope that helps, but I guess it may bring up more questions.