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For those times you'd like to show the 'time ago' of the last comment, if any, on a node, use the following snippet. Goes well near the 'print $submitted' statement.
<?php if ($node->comment_count != '0') {echo t(' - Last comment !interval ago', array('!interval' => format_interval(time() - $node->last_comment_timestamp)));}
Hat tip to dekita for helping me sort this out.