Andrew Wheeler has put together a list (from Publisher's Weekly) of the best selling genre books of 2008.
So, if you want to know what people are actually buying, check it out.
As an alternative, I'd be quite curious to see what readers might have considered the "bestselling" SFF books to be and how those sales figures stack up. I suspect the won't stack up well at all.
107,000 copies of Ender in Exile.