I’d like to use Instruments to peek at some third-party applications on my iPhone, is that possible? If I attach Instruments to some of the applications I develop, everything goes well. But when I try to load some other application, Instruments complain that they can’t “get task for pid XY.” Is that a security measure?
This is the iPhone telling you that the app you're trying to run won't let the debugger attach to it. Without a debugger attached you can't run Instruments.
This is an intentional security measure, withut hacking the 3rd party apps you will not be able to view them in Instruments.