Fall for iPad, summer for iPhone.
All iPhones will get 4.0 but only 3Gs and iTouch 3rd Gen will get Multi-Tasking.
Jailbroken phones with Stacks v3 or SBCategories, Bosspaper or Winterboard, and backgrounder with Kirake or any other manager for backgrounder all do the same things.
As for a better backgrounding in 4.0, I think there probably is. From the sounds of it, I haven't tested the SDK yet, they have a special API for running certain types of backgrounded apps.
So Pandora basically creates it's own process JUST for the music. Combined with a better save state feature that Apple is toting this will let you exit the app which will unload all the navigation buttons and GUI from your memory and save the state. The audio will still play in the background.
For VoIP they will be doing the same thing. Most likely like a daemon process. It'll allow it to be running in the background as a smaller process than the full application and then send a local notification to your OS and you can answer your calls. When you switch apps it goes back to the minimal process and kills the GUI and lets you multitask. When you navigate back it reloads the GUI.
The current implementation with Backgrounder leaves the entire app running in your memory. GUI and all.
So yes, the 4.0 implementation is better. Will I get it? No I have a 3G and I have no intention of updating to a 3Gs. Unless the next iPhone has great specs I'll be getting the EVO 4G which already has these features.