iPhone OS 4.0 is on my phone!
#4
I just downloaded it too. I noticed that with the multi-tasking it keeps apps "open" until you "close" them by double tapping the home key. I have a feeling that forgetting to shut down apps all the way could drain the battery more quickly then it normally does...
#9
For old, as of yet un-updated apps, it's merely the equivalent of a home screen icon. For updated ones not using the other background services, all it does is shortcut you into the last saved state of the app (when you left, or if applicable, when a task was finished).'
If you want a "secret" gem to find, scroll to the left in the multitasking UI to access iPod/background audio ala Pandora, Wunderadio, The Current (the only public radio to really care about) etc. via the audio widget, as shown here:
http://www.youtube.com/watch?v=s2RaA...layer_embedded





I can see some differences on the interface, but not too much. 
Will up date late today and report back