One of the common criticisms of the iPhone is that it doesn’t allow for programs to run in the background. This is, of course, not really true. While Apple doesn’t allow just any program to run in the background (citing battery life and speed concerns), several of the built-in programs do work in background. Examples are the iPod program (you can listen to music while doing other things), the phone (keep a call going while you’re using other programs), mail (can download mail regularly), calendar (for reminders), etc. With iPhone 3.0, this feature has been added to Safari as well.
Let’s say you have a slow loading web page. Now you can go do something else while it loads – that can be reading another page, or using a totally different program.
This is probably a greater advantage on the older iPhones than the new 3GS, since the latter is quite speedy at opening pages. Still, a nice addition.