For now, Tumblr backlinks are do follow, but that will change as more IM people start to spam the site with backlinks.
Whenever you create a blog for backlinks, be sure to include the best quality written content you can as this will lessen the chance your Tumblr blog is flagged/reported as spam.
Don't forget to take advantage of the built in community on Tumblr as they will happily share your content again, which will result in more traffic for your blogs and main site.