Blueloo - iPhone doesn't do true multitasking - they call fast app switching. Apps are frozen in the background and as such won't drain your battery - this is how Apple designed it.
Only very new 3rd party apps will take advantage of the multitasking function - they have to be designed to call upon a very specific set of background functions.
iOS has a central department which looks after background things such as phone calls, push notifications, location services, events etc. As an app is put into its frozen state, it sends a message to the services department telling it what it wants listened out for (for example, a push notification for VOIP), and then goes to sleep. This is to stop all of your open apps sitting active in the background all listening for essentially the same thing, using processing power, memory and battery.
This is why WinMo devices start to get pretty clunky and slow when you have a lot of apps open at one - they are designed in much the same way as desktop apps in that they all stay active in the background, using up resources.
AppleInsider | Inside iPhone OS 4.0: Multitasking vs Mac OS X, Android
has a pretty good rundown on how it all works.