Wordpress SEO appears to cause some warnings if you don't use plugins.
They should be turned off by an @, unfortuneatly in some use-cases/php configs that doesn't work. I've made a small patch to fix that issue - it checks if the directory exists before it tries to open it. You will still get warnings if the directory can't be red, but if it doesn't exist WordPress SEO will ignore it and don't cause warnings.
Build against the latest trunk.