[PhilJ]

Edit /skin/reboot/customer/main/subcategories.tpl

Replace the code at the bottom with... (additional code highlighted in red)

{if $current_category.seo ne "" && $config.Reboot.reboot_category_descr_2 ne ""}
  {if $config.Reboot.reboot_category_descr_2 eq "default"}
    {include file="customer/main/subcategory_descr.tpl" field=$current_category.seo br=true nodialog=true}
  {elseif $config.Reboot.reboot_category_descr_2 eq "truncated"}
    {include file="customer/main/subcategory_descr.tpl" field=$current_category.seo truncate=500 br=true nodialog=true}
  {elseif $config.Reboot.reboot_category_descr_2 eq "scroller"}
    {include file="customer/main/subcategory_descr.tpl" field=$current_category.seo scroller=300 nofade=false br=true nodialog=true}
    {/if}
{/if}

See also template settings > catalog > "Category description 2" to change display type.