on /common_files/customer/main/subcategories.tpl change
Code:
{if $current_category.description ne ""}
<div class="subcategory-descr">
{$current_category.description|amp}
</div>
{/if}
to:
Code:
{if $current_category.description ne ""}
<div class="subcategory-descr">
{$current_category.description|amp}
</div>
{include file="customer/main/login_link.tpl"}
{/if}
Styling it different shouldn't be hard either. Just include the parent div class on your style sheet. for example
.subcategory-descr a.whatever {
}