Since this solution does not apply anymore:

This can be done by re-ordering countries in the countries.xml file located in /components/com_community/libraries/fields/ directory
Just move United States to the top of the list. It's a simple cut & paste operation.

Hi,

you can overrides the template:
copy profile.php
from [Joomla Root]/components/com_community/templates/jomsocial/layouts/register/
to [Joomla Root]/templates/[frontend template]/html/com_community/layouts/register/

edit it, and put this code at the bottom of page

<script type="text/javascript">
    jQuery(document).ready( function(){
            jQuery('select').val('COM_COMMUNITY_LANG_NAME_UNITEDSTATES');
        });
</script>

Works like a charm! Thanks