Tumblr will be rolling out ads across their platform, and yes, on your blog, in the near future:
In addition, users can opt out of having ads displayed on their blogs by turning off on-blog advertising in the Settings. Yes: that means that ads on blogs will be the default – effectively allowing Tumblr to monetize its network of over 306 million blogs from 65 million users, unless those users take an explicit action to disable ads. Users will also be able to disable ads on a per-blog basis if they choose.
Tumblr says that ads will appear in three places, including the main page of the Tumblr blog if the blogger is using the default Optica theme for their blog, as well as on the slide-out section on the web, and on Tumblr’s mobile apps and mobile web.