I've downloaded Tumblr on to my iPad 2 and the button in the App Store now shows OPEN.
But when I tap the 'OPEN' button, nothing appears (it's just the same!)
I checked it again after resetting the home screen view so it's arranged. I searched for 'T' and there's no Tumblr!
Maybe it's bit blurry, but there's all the apps which started with T. I've searched in the search Spotlight too but also, nothing appears with 'Tumblr'.