I thought Tumblr was fairly clear in their post about the new site. They’ve redesigned the dashboard in preparation for a much bigger overhaul:
We’re getting ready to push our biggest interface overhaul since last November, and we’d love to get your feedback.
Nearly every piece of the interface has changed. We’ve tried to perfect the presentation of the Dashboard, and the retooled interface will enable a lot of the new functionality we’re prepping to launch.
In other words, the update hasn’t happened yet. But if people are confused on this point, it’s Tumblr’s fault. The Tumblr Beta site doesn’t look very different, and there’s nothing inside to indicate new features might be on the way. It might have been better to wait until the new features were ready before unveiling the public test site. Indeed, that’s what a test site is for: creating new features and functionality and working out the bugs so that you can avoid confusing your mainstream users.
So far, confusion isn’t being avoided.