OHHH man... is this a heavy topic. do a search and there is tons of stuff. i have done my share of research on it and frnkly, until someone does an in'lab test with meters and stuff than there will be no proof for either argument.
but of the people who DO wear them, most will say they have had less problems since.
the funny thing is, when you get down to it and research the definitions of the cirtifications it will show that they ranked for speeds SLOWER than wakeboarding and for hiting rocks and other hard surfaces while kayaking not water impacts, BUT OF COURSE A HELMET SHOULD BE WORN IF HITTING RAILS NONE THE LESS AS SAID!
;)
i got a shred ready Phly on discount... i'll have to see how it works this summer ;) but i never had a concussion
OH and wakeworld has this article on the whole subject too
http://www.wakeworld.com/getarticle.asp?articleid=1499