Remember – search engine spiders are amongst the most ‘disabled’ users of the web; unable to hear, visualise formatting, or imply information from structure or colour. By designing inline with accessibility standards http://www.w3.org/TR/WCAG10/), you are designing for Google.
You can read the whole thing here: http://www.boxuk.com...essibility.html
Of course, there's the next step to consider, and that's actually creating optimised content. To be honest, a CMS can only take you so far (allowing you to create good title tags for each page, ensuring that the correct tags are used, etc.), it needs the good old-fashioned grey matter to do the rest of the work.
The right CMS will put you on the right track, whereas the wrong one will mean that no matter how much effort you put in, your site will remain 'seo-challenged'.