...pity my neighbours, for my shiny new iPad has given me a virtual piano, with which I am attempting to transpose assorted songs by ear.
Also, the main problem with all iPad piano apps is the placement of the hands. If you want to actually play, as opposed to hunt-and-peck typist style that seems to be on most of the demos, then it's really hard to hold the keys - the iPad is either too sensitive, or not sensitive enough, and you don't have the option of resting/trailing your fingers across the keys as you proceed to the next note. This makes the notes sound rather clipped, if the keys aren't side by side. Haven't figured out if there's a way to control that yet.
Perhaps a pressure sensitive piano app? Now that would rock.