I would say there are no such tests/toolboxes, that would allow you to properly conduct any cognitive testing on iPhone, iPad or even using web-based applications. There are some games that attempt to do it, like the one suggested by @Speldosa, but nothing really serious.
At the moment there seem to be no way to control and record different variables (like reaction time) as tightly as with desktop application, like Psychtoolbox or Neurobehavioural System Presentation (you will find plenty of problems there anyway). What's more, iPhone and iPad are tightly controlled platforms, and developers are limited in getting access to some user parameters and data. For example getting informations about parameters like brightness/contrast or volume would be important for cognitive testing apps by default. But it's unlikely to be possible with the current Apple policy (I would bid more for Android devices, as they are much more open for such purposes).
I agree tho that touch panels are promising platform. I imagine people do work on that in some labs, using jailbroken devices, but I am not aware of anything available in the public domain at the moment.
Another issue is novelty of using touch devices. People didn't tested them yet in terms of proper psychophysical accuracy, it's still undiscovered area. Potential seem to be very promising (think eye tracking using iPad camera for example), but we shall see what comes out of it.