I have a diatom filter although I haven't needed to try it during an ich infestation.
I believe the diatom filter will remove the ich parasites, but only the free floating ones, and may not get them all (if they get stuck on something during the filtering for example). However, ich has a 3 stage life cycle; the other 2 stages are on the fish and in the substrate. The diatom filter won't do a thing there.
I have used my to clear up a horrible algae bloom that wouldn't go away, and also used it to clear a cloudy tank that was a result of Onyx sand (even tho it was rinsed, it clouded the tank up completely). Was well worth the purchase IMHO
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.