Indigo, the Mac-based home automation program, officially reached v4.0 this week and has added support for iPhone, too. The Indigo Touch application is a native iPhone app that allows you to control your home remotely. If you're not an Indigo user, but you have an iPhone, Indigo Touch is still a worthwhile download because it can connect to a "demo house" that's fun to play with and wow your friends. The program looks and behaves exactly as iPhone users will expect.
The summary at Macworld is a good place to start your exploration.
UPDATE: Perceptive Automation also has a great screencast that's worth watching. Matt left this in the comments on this post, but it's too good to be possibly overlooked. Thanks, Matt.