It should be
<?php echo $prototype_helper->link_to_remote('index', array('url'=>array('action'=>'homepage')), array('update'=>'DivContent'));?>
although the documentation stated what you used :(
Thanks for reporting it, I've just fixed the documentation bug.
BTW to make it even shorter you can use Sintags like
<%= link_to_remote 'index', {:url'=>{:action=>'homepage'}}, :update=>'DivContent' %>
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