Tumblr have just updated their iPhone app to use native iOS code. Following in the footsteps of Facebook and their much improved app, this most probably spurred on Tumblr to do the same.
The design of the app hasn't just changed much at all. It's mainly the same design with a few improvements here and there. The home tab now has images which are noticeably bigger, they're very like how Facebook displays images on its app.
The notification panel has also been tweaked. You can now see a thumbnail of the actual post that has been reblogged or liked. This is even better than their desktop website which simply says a post has been liked or reblogged and the only way to find out what post it is is to click through to it.
Unfortunately there still is no iPad app. Who knows when an iPad version will come, we just hope it's soon.