@geoffreyconley well, it is still very customizable to just be a simple blog. I think it's not going to hurt WordPress, after all, they are porting the front end of Tumblr to the backend servers of Wordpress.
They claim that users shouldn't even notice a difference.
If you're using either one, it doesn't seem like they will cross paths in anyway that's noticeable. At least that's what they claim.
I guess we can only take them for their word and see what happens.