Great info as always from Joost de Valk at Yoast.
<a href="http://yoast.com/wordpress-seo-url-permalink/">WordPress SEO URL / Permalinks considerations</a>
Let's answer some of the questions that continually pop up: what does the optimal WordPress SEO URL / Permalink structure look like? How should you use your URL's, what should and shouldn't you do with them. A lot of questions below, a lot of answers, most with a reference to a video or post from Matt Cutts, the head of Google's web spam team.
The perfect WordPress SEO URL structure
I'll be honest. I think it's dead simple and that you basically have two options. The URL should end with the post name and could possibly be prefixed with the category. No other options really make sense.
This means you'd end up with a permalink structure as follows: