Filter the seperator arg for the tag cloud function. Put the following in your theme\'s functions.php

/**
 * Filter arguments of tag cloud widget to enlarge our text and
 * add commas
 */
function myfunc_filter_tag_cloud($args) {
    $args[\'smallest\'] = 18;
    $args[\'largest\'] = 32;
    $args[\'unit\'] = \'px\';
    $args[\'separator\']= \', \';
    return $args;
}
add_filter ( \'widget_tag_cloud_args\', \'myfunc_filter_tag_cloud\');