Here are three interesting essays dealing with the relationship between Willow and Tara on Buffy the Vampire Slayer. Two by Todd R. Ramlow ( First | Second ) and one by Andrew Gilstrap. (Sequentially, it should be Ramlow 1, Gilstrap, and then Ramlow 2.)
Personally, I tend to lean towards Gilstrap's assessment. While I can see some validity in the witchcraft = lesbianism, it just seems as though Ramlow is looking way too hard for a connection that doesn't really exist. I've always agreed with the camp that equates witchcraft with drug use/abuse, because the parallels are just entirely too obvious.