Today's Walt Mossberg column in the Wall St. Journal is a review of the iPhone 4.

He notes that "multitasking" on the iPhone actually (often) means that apps are put into a state of suspended animation, so they aren't really "running" and eating up the battery life.

To see which apps are "multitasking," you double-click on the round button. You will see 4 apps lined up across the bottom, and if you scroll them sideways you will see a lot more...in my case, 17 apps. I was able to turn a couple of them off by holding down the app until they begin to quiver, then selecting individual apps. However, most of them don't seem to want to go to sleep...like Hal from the movie 2001.