It's not the multi-tasking you see on PCs. It's pointless to have multiple apps running full steam when you can only see one app at a time.

iOS 4 does provides additional services for 3rd-party apps to run in the background - for example fast app switching, or background audio, or push notifications. But iOS does not automatically make 3rd-party apps run in the background - the app's developer itself needs to make use of those background services that iOS 4+ provides.