I've seen umpires knocked to their feet at first class matches (pretty sure I saw an international umpire knocked down on TV too), just like I have seen fielders knocked over. Personally I wouldn't think it should be required to wear a helmet, but probably in these days of health and safety it would be wise for protection to be available to the umpires should they wish to wear it.
Yeah, that was Billy Bowden in the 06 Ashes. Geraint Jones I think swept hit and hit him at square leg.