Honestly, the iPhone/Touch is a poor supplement to a real planner. For one it doesn't have some planner essentials like a task list or to-do list. Also, it doesn't have the ability to add custom apps that may help you customize and optimize your planning, which are readily available for platforms like Palm OS.
At this point they're just not a good replacement, but hopefully 3rd party apps will change that. The SDK comes out in February and there's no telling exactly how long it will take for developers to come out with any apps, let alone some quality apps. If I needed a full-fledged planner very soon, especially for business, I would probably go with a RIM (Blackberry) or Palm product. There are various programs which make it possible to sync these devices with you Mac.
Since you're posting in the Apps and Games subforum, I guess you could also be talking about an app on your MAc. If that's the case, I really like Entourage. Unlike the Mac programs Entourage brings all your communication and organization apps under one roof, which I really like. Plus, unlike it's cousin Outlook, Entourage has a feature called Projects which can really help you organize any kind of project or major event. I use it to organize my grad school classes.