Google is loosing his trust more and more !!
there is too many sites using this method and they rank pretty good in your search engine , even more they have stolen content from other website .
i can give many examples about those sites if u want .
others are using paid link from high pagerank sites in footer and they have now pr 5 within 6-8 month with last Google update , example can u please check this sites backlink strategies a9d2.com , girls-ly.com , almastba.com ( i have too many urls also ) , this site in using this code
<div style="display:none">
<a href="http://www.a9d2.com/" title="شبكة اصداء"> شبكة اصداء
</div>
in too many high pr site
let me know , please regards