Will Norris put together a nice WordPress plugin to place Open Graph metadata on the page. Today I patched it to address a few bugs I and others have found.
The patch switches functions that depended on globalizing $post to use $wp_query->queried_object and similar.
opengraph_default_url() is changed to try get_permalink() only when is_singlular() is true. Otherwise it uses the blog’s base URL. This isn’t perfect, but it’s better than having the front page and all tag/category/archive pages report their og:url as being the permalink for the first post on the page.
As suggested here, I changed the opengraph_default_description() to use the post’s excerpt if is_singular() and the post includes an excerpt.
I changed opengraph_default_image() to test if the theme supports post thumbnails before calling has_post_thumbnail() to avoid the Fatal error: Call to undefined function has_post_thumbnail() errors.
I submitted it as a bug report in the plugins Trac, but I don’t check there for tickets on my own plugins, so you might apply the patch yourself.