Tumblr is becoming pretty useful for SEO as you can redirect the profile you have made to your website URL. Whenever you find a high PR profile to 'reblog' or 'like' these become back links which are DO FOLLOW.
http://www.tipifam.com/internetmarke...acklinks-free/
I stumbled across this and thought I would share it, it tells you step by step what to do as well as some high PR back links at the bottom of the page to get you started!
You can repeat the process on as many different blogs as you can find with a high PR ranking and create back links quickly