Would be interesting to see if you get the same YSlow message when checking your site without any plugins. Its a little difficult to be sure because of the minification of HTML/CSS on your site, but it seems some of the plugins are not inserting stylesheets in to the <head> part of your document, and placing them when they appear instead.
From my understanding of YSlow’s documentation http://developer.yahoo.com/performance/rules.html#css_top, it helps to ensure progressive loading – or in other words, the visual loading of the site for the end user is enhanced.
If a fresh install of your theme (perhaps on a test site) results in the same YSlow message, then feel free to ignore the above. If not, perhaps activate each plugin and run the test to see which ones are adding them in erroneously – then maybe drop the plugin authors an email.