This isn't new but I just discovered it. There's a mobile formatted version of every Tumblr blog. To see it, just add "/mobile" to the end of a Tumblr blog's URL. There's doesn't seem to be any mobile browser detection and redirection, you have to manually change the URL.
A few examples from Social Media Explorer's list the Top 25 Tumblr.com Blogs:
Tumbler is a free microblogging service that lets users post text, images and video or audio clips. Like Twitter, Tumblr users can follow other users and see all followed user's posts in a timeline. Other social media features include the ability to "like" or "reblog" another user's posts. Unlike Twitter, there is no limit to the size of a Tumblr post.
It looks like Tumblr's reformatting for mobile strips out background images and Flash content, resizes images to 250px wide and changes text to black on a white background. Pagination, at 10 posts per page, is unchanged from the mobile version. The mobile versions of some image heavy Tumbler blogs I looked at ranged from 400 to 900 KB in size versus 1400 to 3000 KB for their desktop equivalents.
Via: MobileViews Blog