where is .entry-utility used?
*sigh*
Again, from loop.php:
<div class="entry-utility">
And again, from loop-single.php:
<div class="entry-utility">
All of the "Posted in" markup is inside of this DIV.
If you want to target the "Posted in" container directly, note these classes:
<span class="%1$s">Posted in</span> %2$s', 'twentyten' ), 'entry-utility-prep entry-utility-prep-cat-links', get_the_category_list( ', ' ) );
That sprintf bit produces this:
<span class="entry-utility-prep entry-utility-prep-cat-links">Posted in</span>
So, you can target either of the following classes:
.entry-utility .entry-utility-prep {}
.entry-utility . entry-utility-prep-cat-links {}