See these posts:
- http://wordpress.org/support/topic/instead-of-seeing-the-actual-post-title-for-pages-i-see-the-text-post_title
- http://caspianit.co.uk/all-in-one-seo-post_title
Someone says there:
If you use All in one SEO, you need to set Page Title Format: %page_title%. Your page title will be displayed properly.
I have not tried it myself