I just came across this today and felt I had to share this with some of the other iPhone owners on the forum. You must be jailbroke to install this but I suppose that's a given.
The application is called ProSwitcher.
First thing you need to do is update your repo and add this source.
This Multi Task app is sort of like the palm pre. I've got mine set where I just swipe down from the task bar to background a task. Then I can swipe between all the open applications. You can go into settings and change that to a tap or other swipe gestures. I highly recommend checking it out if your jailbroke. It's currently in Beta and it is a free download.
Here is a video of it in action. Note the phone in the video is running a Palm Pre theme.