Is Fast App Switching going to be automatically updated without any active developer changes to the App? Or, does the developer have to use a specific API to enable Fast App Switching?
I think there's 2 parts to that question. As I understand it:
A) Developers will have to update so that the app's state is saved so that 'fast-switching' returns you to the exact place you left off. That won't happen until they update. That's a critical part of fast-switching, so you can say that the developers have to update.
But...
B) The way that apps show up in the quick-launcher and can be jumped between each other will happen for any apps. (Even though this is more of a graphical thing than a performance thing it's still part of what people will think of when they think "fast switching.") So you'll get that quick animation for everything, but without the saved-state it may or may not be helpful depending on the app you're using.
EDIT: For a sort-of example of what I mean by point "B," watch at 0:50 here:
http://www.youtube.com/watch?v=35UuWv6JRYg
They're switching to Safari. Now, Safari probably has been updated, obviously, but imagine if it hadn't been.
It would still look exactly like that when fast-switching. Safari is an example of an app that normally brings you back to where you left off, so it's an example of an app that would essentially give you fast-switching
without being updated. You probably have a few other apps like that...
they won't really be fast-switching, but you'll feel like they are.