The OP wants a single application that runs on both the Mac and the iPhone or at least a Mac OS X application that is compatible with an iPhone application. This means that he must identify an application available at the iTunes App Store that is compatible with an application available at the Mac App Store. If the OP were looking for a MacOS X application, then I would enthusiastically recommend
OmniPlan. I understand that
Merlin is also good. If he were looking for an iPhone app, then I would ask him to give serious consideration to
Project Planner. Unfortunately, he wants a single application that is available on both Apple platforms. This is no such application.
