The folks at Overcommitted have made the official iPhone app for Highrise. It’s simple and effective, as one would expect any 37signals product to be. I like that the app’s main toolbar matches itself to the color scheme you’re using with the browser-based version. They’ve also made the initial synchronization fun, but I won’t give away how.
Highrise lets you manage communications you’ve had with contacts and customers as well as tasks and appointments. I’ve been a happy customer for years. Look for my review at TUAW this week.