the problem is not the multitasking, it's the users and developers
If I have an app that does not support iOS multitasking, when I close the app via the Home button, the app is closed. end of story
if i close an app that support fast app switching, the current state of the app is saved to FLASH memory (the 16 or 32GBs of storage). When you go back to the app, it loads the saved state back up.
If I close an app that supports multitasking, such as Pandora, if Pandora is playing a song it will continue to play that song until another music app wants to take over, like the Apple iPod app.
There is not enough multitasking apps that use all aspects of Apple's multitasking API to bog down anything, and even if there were, I doubt the speed loss would be very noticeable.
Now running apps in the background using the jailbreak app Backgrounder, THAT is a huge waste of both active memory and battery life.
I think we all need to wait for the apps we are using to update so we can "multitask".....I agree this is not true multitasking and programs are not running in the background. I guess it would piss me off if I was playing a game, had a high score, and had to start over......
Again, apps have to be updated to support that. They COULD have already supported that well before iOS. Field Runners, ever since iPhoneOS 2, saved its state so that if it crashed or was quit out of, you could resume immedialty after opening the game again. This is nothing new, developers just don't put the effort into that. This is why the APIs are now around, to make it easier to add these features.
What the AppStore needs is a key that shows what Multitasking features the apps support. If it supports Fast App Switching, have an icon on the store page that denotes that. If it supports background music playback, show that too.