I am not a big fan of using/abusing UIAlertView for text and login input but if you choose to use it for that, Keith Harrison has a look at the changes in iOS 5 that mean that at least you don't have to go hacking at the view hierarchy any more.
Rodrigo Sieiro with a small extension to MKNetworkKit (which I linked to last week) to allow easier OAuth authentication. Anything that makes OAuth easier is surely a good thing.
The new Twitter APIs in iOS are iOS 5 specific, but if you are not quite ready to make that commitment with your app then you might want to check out this library which provides look alike Twitter sheet functionality on iOS 4+.
Johann Döwa links to a couple of open source rotary controls for iOS. I like the look of the one from Domestic Cat software, pretty.