The three finger swipe up would be the same as you do on a Macbook's trackpad on Lion.
The reason for iPad exclusivity is because the iPhone's screen is too small for such. iPhone Multitasking might stay the same.
What are the apps you imagine it displays? 5 most recently used? Only apps with background tasks running such as music or location? Because multitasking on an iPad and Mac are very different. I just don't see it working on the iPad since you don't leave windows open that you can see in a mission control view.
If iPhone multitasking stays the same functionality-wise, maybe they could update the look so we get live previews? Just to spice it up a bit... this could show up after double-tapping Home while in an app.