To clarify, when you attempt to access the main index page (http://<server>:<port>/<context>/index.jspa) you are being redirected to the login page or is this only happening when trying to go to the URL for specific posts?

The posts that you see when you are logged in, are they from a personal, community, or system blog?

Can you send a screen shot of the global blog permissions page from the admin console under Blogs -> Permissions -> Global Permissions?

For one of the blogs that you are not seeing posts listed from when viewing the main page as an anonymous user, can you check the permissions for that specific blog?

You can do this in the admin console by going to Blogs -> System Blogs (or Blogs -> Personal Blogs if the blog is a personal blog) and then clicking on the "Edit" icon for the particular blog in question.  This will bring you to the edit page for that blog and near the top of the page will be a link labeled "Edit Blog Permissions."  Click that link to see the permissions for that particular blog.  Can you send me a screen shot of that permissions page for the particular blog that is not showing posts when viewing the main page as an anonymous user?  The main concern would be if the Read permission is not granted for the "Anyone" line.

What version did you upgrade from?  Also, what type of blog is the blog in question - a personal blog or system blog?

This appears to be a bug in one of the upgrade tasks that would happen between the 2.5.5 and 2.5.7 releases.  I've reproduced locally and logged this as a bug.  The id of that bug is CS-12370.

The current workaround would be to do what you have already done in granting read access to the anonymous user for the specific blog.

Would it be alright to make this case public to benefit other community members who may encounter the same behavior?