You're right MadridMan, some stores (especially party stores) do promote it and bars have lots of parties to liven up business. Heck you can even find pumpkins. I love them because they come with a sticker on them explaining how to make a jack-o-lantern. Some even have stickers of eyes, a nose and a face! But commercial interests aren't the only reasons.
Celebrating Halloween has become quite popular in schools thanks (or no thanks, depending on your perspective)to English teachers (like me) using it as a cultural theme in their classes. It's even becoming big in British schools even though England doesn't tradtionally celebrate it. It's a fun way of teaching English and creating a fun atmosphere. BUT I can assure you it isn't widely appreciated by many people who feel it is an American custom encroaching on Spain. I try to tell them it's a 3,000 year-old tradition which probably started in Ireland, but that doesn't seem to make a difference.
But it's so much fun for kids, it's only normal that it catch on.
I go through this debate every year, but it's useless, because no one agrees. But that's all right, 'cause I can understand both sides of the argument.
So to answer the question: Halloween is not celebrated at all in a serious way by most Spaniards, and it's even rejected by some. But it is catching on especially among young people.