Wait - I see two buttons on all Tumblr sites: one that says "+ Follow" and one that says "Join Tumblr". Is this something that I'm in a bucket test on or does it go away if you're logged or something?
This isn't meant for advertising though (these are existing users after all). Those two buttons allow you to:
a) Follow that Tumblr (like RSS)
b) Go to your Tumblr Dashboard