Hey all,
I just came across this. According to a conference being held by the Ecological Society of America on reef restoration:
Vermetid snails reduce growth and survival of corals and shift coral species composition. Affected species include Pocillopora, Montipora, and some Porites species primarily.
More info here:
http://eco.confex.com/eco/2007/techprogram/P7313.HTM