WooThemes is apparently pulling in $2+ mil per year according to the Mixergy interview. (http://mixergy.com/woothemes-adriaan-pienaar/)
The market is quickly getting very crowded but I'm currently in the process of selling premium WordPress themes myself, mainly at the request of clients who prefer a cheaper solution that is easier for them to customize. If there's interest I could blog about it or provide some insight into my research (and development) so far.