I have no experience with iOS development but I do with Android development. I was able to integrate the PI System with Android through PI Web API, which is a RESTful web service on top of the PI System. It is currently on its beta version but it will be released soon. I am sure iPhone is compatible with this type of web service since it is very common in the industry. You can download it from the vCampus Download Center.
PI Web Service (SOAP) might also work but it is not our recommended product in case you are starting a new project.
I am not sure about PI ActiveX control.
Hope it helps!
There is pretty much no chance of getting an ActiveX controls to work on the iPhone, the technologies are based on different platforms and are more than a decade apart when first introduced.
There are ways to get PI on an iPhone as Marcos posted, with PI Web API being a good choice. That product is still in development, but you can write you own in the mean-time. I have a blog post that shows how to do that, you can find it here: