If you want to make your the author’s comment stand out from the rest, simply adding a code to check if the commenter’s email is the same as the author won’t work. It has some flaws like what if one of your readers know the author’s email address? They can the blog author’s email address in their comment so their own comments are highlighted.
To fix this flaw, write a code that will check for the author’s email address and check if its the user id of the blog owner. So, only the author’s post are highlighted even if other readers use the author’s email address.
Read the tutorial here: http://www.mattcutts.com/blog/highlight-author-comments-wordpress/