Wordcastnet have published a great article, “How To Use Internal Linking in WordPress 3.1.”
Using AJAX and jQuery instant functionality, the Link function in the Visual editor now includes a search through your post content to find that internal link and quickly add it to your post.
Full post: http://wordcastnet.com/2011/how-to-use-internal-linking-in-wordpress-3-1/
Internal Linking in WordPress 3.1 Codex: http://codex.wordpress.org/Version_3.1