Both biro and yeshuaisiam are correct, depending on one's point of view. If one views Satanism as an organized religion dedicated to literally worshiping Lucifer aka the Devil aka Satan in a blatant and conscious attempt to take glory away from the One True God, then Wiccans are not Satanists. They are, at best, post-Christian polytheists who are very much searching for the Truth but because of their own insecurities, oftentimes (at least in the US) founded upon some sort of spiritual disappointment in Christianity, refuse to look at Christ as THE Truth. I've known many Wiccans in my time and have yet to meet one who is actively trying to do evil in service of the Devil. I have also met Satanists of both the LaVeyan crowd and people who truly believe they are in league with the Devil and he speaks to them. The latter are very strange and I don't associate with them at all if I can help it. The former are just atheists with an attitude problem.
However, if one views Satanism as yeshuaisiam does, as any sort of refusal to give God His due glory, then Wiccans are Satanists.
But I would add, so are most Christians, myself included. And that's not some pretentious "I am chief among sinners" self-righteous drivel. I might as well be sacrificing a goat to the Horned One the way I've been carrying on lately.