I seriously doubt this update will magically make all apps multitask - Apple likes much more control than that, and they don't like the user experience to be compromised by having to manage background processes etc. If I were Apple, I would do it like this:
1. Apps would have to be specifically developed and submitted to the app store to get "permission" to multitask. If an app did not need real multitasking (e.g. if it can just "save state" on quitting) it would be rejected.
2. Clever APIs would minimise the need for real multitasking (e.g. a Calendar API that allows apps to issue notifications at a predetermined time).
3. Apps could have a secondary "mini-app" for multitasking purposes (e.g. a music streaming app would have the bare minimum for music streaming running in the background, then open up the full app when it is selected again).
4. Apps would be placed in categories; for example, if you open an app that plays audio, any audio app already running would quit.
5. There would be some provision for the OS to pre-emptively save the state of an app when quitting, again, to 'simulate' multitasking.
These are, of course, back-of-an-envelope ideas from a non-programmer, but my point is that there are all sorts of ways to get the *effect* of multitasking without having to go the whole hog (push notifications achieved that for some apps already - I am just predicting they will go further).
Yikes! Background processes are not the same thing as multitasking. Multitasking is when multiple apps are open at once. Background processing is when an app continues to run even when it is closed by the user. This is when a task manager is required to make sure apps that have been "exited" really stop running.
You can implement multitasking (for any app) without implementing background processes. A process manager is not necessary - only a simple way to switch between open apps (like Safari pages, for example).
1) The vast majority of apps do not use much processing (or battery) power while they are idle, so there is no need to "clear" an app for multitasking.
2) Actually, this is needed in addition to multitasking, not as a replacement. Apps should really be able to update their status without launching them or keeping them open for long periods of time. Take, for example, a task managing application. It would be nice for the phone to automatically notify for tasks that are due and update badge icons accordingly. As it is now (and would be with only multitasking implemented), the app must be launched in which case tasks can be missed entirely.
3) This is also not really necessary as as the only difference would be the rendering of graphical elements, which idle apps would rarely do. There are exceptions, such as Pandora, which would update album art, but that would be a drop in the bucket compared to streaming audio.
4) Something to that effect; it certainly is senseless to run the iPod and Pandora or Video simultaneously.
5) This is how the OS already behaves. It's just a poor substitute to, as you say, simulate multitasking. The thing is, multitasking adds almost no overhead to the processor. As long as there is enough RAM, there is no reason not to implement full multitasking.