Apps running? or not?

Regarding having an app open and then pressing home. What happens? I've heard that it runs in the background unless you press and hold home to quit it properly. However in an iPhone Dev video one of the evangelists says that only one app can run at anyone time, and as soon as you press home or start another app, the original one closes. It's also stated that apps should "save their state on close, so that when they are re-opened, the user is returned to where s/he left off". This also indicates the app is removed from memory and closed properly.

Then, in another dev video, a different evangelist says that all apps should have an event handler that responds to a "free up some memory" event. That is, should an app require some more memory, it can send out this event and all apps that can handle this event will recieve it, and will do whatever it is they do to free memory. The browser for example closes any multiple open windows etc.

Now what bothers me here is that one person says that the apps are closed when not visible, and the other, by saying that an app can respond to an event, would indicate that they are still running resident in the background (well, not running, but still resident).

Before I actually email one of these holy evangelists I would like to know if anyone here can shed some light on the matter for me and confirm one or the other.

Thanks

p.s. These are the videos available on the official iPhone developer site