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//Gets post cat slug and looks for single-[cat slug].php and applies it

add_filter('single_template', create_function(
	'$the_template',
	'foreach( (array) get_the_category() as $cat ) {
		if ( file_exists(TEMPLATEPATH . "/single-{$cat->slug}.php") )
		return TEMPLATEPATH . "/single-{$cat->slug}.php"; }
	return $the_template;' )
);