Simply titled WPTumblr, the app was created by Ad Astra Consulting which allows users to reblog, like and catch up on Tumblr blogs that you subscribe to (aka follow).
Thus far the Windows Phone 7 app supports all 7 Tumblr formats and allows users to upload images, although unlike the official Android, iPhone and Blackberry brethren, Ad Astra’s app does suffer from one critical flaw.
New Post Page screen capture. WPTumblr supports all seven native post types. Currently you can only submit music/videos via URL since the WP7 framework doesn’t allow direct access to your music or video libraries. (Official WPTumblr Blog)
Ironically this limitation may convince Tumblr (the company) to wait until Windows Phone 7 matures a little bit before creating an official app (as all of Tumblr’s official mobile apps support video uploading).
Currently Ad Astra Consulting is selling WPTumblr for $0.99 within the Windows Phone Marketplace, and as far as I can tell they are the first company to launch a blogging platform for WP7 (note: WordPress is rumored to be working on an app for the platform as well).