One nifty little iPhone feature that left an impression with me is the way that the keyboard in the "Messages" application slides in and out with the keyboard, as if it were attached to it - but it isn't. I needed a similar feature for an app that I'm currently working on for a client, so I decided to investigate how to implement this feature in my own stuff.
Viewing messages, keyboard not shown.
Keyboard displayed. The toolbar moves up, attached to the keyboard.
Although my first instinct was to simply use an implicit animation using UIView's +beginAnimations:context: method, I decided to quickly search the web for some information on how other developers approached this problem.
The results were a little surprising, possibly because the proper way to do it wasn't as easily available before iPhone OS 3.0 - various people on different message boards and mailing lists suggested a hack that involved looping through all instances of UIWindow to find the one containing an instance of UIKeyboard, a private class that isn't even documented in the iPhone SDK. Those hacks then proceeded to modify the dimensions of the view that contains the keyboard and adding the UIToolbar instance as a subview, so it would animate into the key window right along with the keyboard.
Needless to say, these hacks are not exactly future-proof or what I would consider a proper implementation of this feature. The Core Animation route was the one to go, so I proceeded in that direction.