This app requires a jailbroken iPhone, but it's pretty neat. Multiflow brings Expose-like functionality to iOS 4.0.1. Installation should be familiar to anyone with a jailbroken iPhone. Enter Cydia and search for "Multiflow." It is a paid app, so bring your US$4.99.
Once installed, you'll want to select an activation method. There are several options, like a double press or press-and-h