Update January 18th 2012: A new and easy to install threaded commenting hack version 2 includes this hack's functionality, you might consider using it.
There are some highlight author comments hacks out there, but most I've seen check only that comment author name is something. It is better to check the author url (so anonymous user's won't disquise as you so easily)
Edit your template html. Select Expand Widget Templates. Find line:
<b:loop values='data:post.comments' var='comment'>
After that add:
<b:include data='comment' name='commentDeleteIcon'/>
</span>
</dd>
(you can search using the first line). Just after those lines add:
</div>
Then save the template. Now we need only to style the admin-comment to look different. Go to Template Designer, choose Advanced, Add CSS, and add following:
.admin-comment {
background: #F4F4F4;
margin-top: 6px;
margin-bottom: 6px;
padding: 3px 3px 0 3px;
border: 1px solid #EDEDED;
}
And then to the testing. You can style admin-comment background otherwise, if you like, using CSS. I have installed this hack on this blog.