Here is great interview about WooThemes, how they market it, build and sell it. Many great ideas and lessons to learn. I was very surprised to hear that many people buy premium templates.They have more than 40 thousand customers according to their website. An average theme is 70-100$.
Update: They are now focusing on wordpress only by adding e-commerce plugin for free and selling additional plugins for e-commerce. Reason they focus on wordpress is big distribution. Also there is plugin search in wordpress that most bloggers search in order to add new functionality to their blogs or improve existing ones. Being listen on that internal search is great traffic driver as well. If you build some software you can consider building it on top of wordpress because of this factors.