Look here CrackBerry straggler! I kid, I kid...
I am serious though. It is irritating that because Apple doesn't allow 3rd party apps to be run in the background, everyone assumes that is universal for the entire phone. SMS, Mail, Calendar, the iPod, and the Phone all perform background processes.
If nothing ran in the background, you wouldn't be able to listen to the iPod while you use every other feature of the phone (except talk that is). If nothing ran in the background, you wouldn't get notification of new text messages, phone calls, and emails while browsing the web, using Notes, checking the Weather, etc.
The iPhone doesn't kill everything either, it suspends certain things, like Safari. If you are browsing in Safari and have 4 separate windows open, all loaded to a different page and hide it to go to the home screen, it will suspend your positions. When you open Safari again, it will still be just like you were looking at it before (it may take you back to the top of the page though, I can't recall).
This is why I get so annoyed when people b*tch and moan about no multitasking. They are completely missing the concept of how the iPhone handles these functions, and that while it may not be true multitasking, it is hardly detrimental. Especially in the case of instant messaging...
One of the biggest complaints is that you can't have an IM app without allowing it to run in the background. Apple's push notification system will clearly kill that problem, and it isn't like you can IM AND use the browser at the same time on any other phone anyway. Simultaneously that is.
For example, if you are using a BlackBerry Curve to browse the web and an IM comes in, you will get a notification. You now have to minimize or close the browser and open the IM app to see the new message. When you are done, you have to minimize the IM app and go back to the browser.
With the iPhone's push notification system, you will get that same new message notification, and you will press home to get to the home screen. Now you open the IM app, and you will see your old conversation with the new message all the same. You now press the home button, press Safari and resume browsing right where you left off.
It is the same thing. It requires the same number of button presses on the iPhone as it does on the BlackBerry, but minus the crappy menu scrolling. The only real difference is that the iPhone handles it in a way that doesn't sop up system resources. It is also universal across the other features of the phone, I just chose the browser because that is probably the feature most used.