booleanScallop.scala

  object Conf extends ScallopConf(args){

		class BadArgsException(msg: String) extends Exception(msg) {}
		
		val properties = props[String]('D') andThen
		(_.map(_.toLowerCase)) andThen
		(s => s.map { case s if(List("true","t","y","yes").contains(s)) => true 
			          case s if(List("false","f","n","no").contains(s)) => false
			          case s => throw new BadArgsException(s + " is not a boolean value")
			        }
		) 
	}

One suggestion - if your BadArgsException would extend ScallopException instead of simple Exception, you would get a bit better error output - at least, the user wouldn't see the unneeded stack trace.