Matt Mullenweg, writing on his photomatt.tumblr.com blog:
When the possibility to join forces became concrete, it felt like a once-in-a-generation opportunity to have two beloved platforms work alongside each other to build a better, more open, more inclusive — and, frankly, more fun web. I knew we had to do it.
I love this, and truly believe the open web is a big part of this.
In the underlying technology of our platforms, I think there are some good opportunities to standardize on the Open Source WordPress tech stack, but the front-end user experience on Tumblr will evolve on its own path.
Expected and good news. My guess is that Tumblr has not been working on mayor back end stuff the past few years, and somehow standardizing on the Wordpress stack makes a lot of sense.