@nevster,
The best permalink structure for seo is /%category%/%postname%/
This permalink structure gives you the most keywords loaded into the URL of your post. Since the category that you’ve placed your post under usually relates to the post title, you will have an extra SEO benefit if other websites use that permalink structure to link your post.
EDIT
After reading over the link Mike shared below I felt I should also mention that starting the permalink string with %postname%, %category%, %tag%, or %author% will invoke WordPress’s “use_verbose_page_rules” which will cause the rewrite matching system to generate individual rules for every Page, then check those first. If you have lots of pages, this is a big hit to the system. The performance hit happens at about the 50+ page mark.
In order to have the ability to scale your site on a larger level you should start the permalink structure with a number like: /%year%/%category%/%postname%/
Your pages will always use the /%pagename%/ permalink structure any time you enable permalinks so the /%year%/%category%/%postname%/ will only be applied to your posts.
If your site is more CMS than blog the optimal SEO url structure would be to use child pages where the parent page could be considered a category.
For example a university website might have a parent page “Campus Offices” with child pages of “Business Affairs”, “Athletics” etc.. Then URLS would like this: www.university.edu/campus-offices/athletics/ which is great for the user experience and seo.