Push...
When connected to an Apple server, messages can be sent to the Touch that can indicate activity...even for apps that aren't running. For instance, a message can be sent indicating that your IM app (though closed) has activity directed to it. Apple considers it a compromise rather than allowing apps to truly run in the background.
Apple took heat (fairly, IMO) for limiting functionality that prevents things like IM from running while you're doing something else. I've heard jailbreaking allows full background apps but I don't know first hand.
This approach will reduce power consumption, but it is still a disappointment. The only way push is useful is when you're connected to a remote server. I can imagine all sorts of uses for having local apps in the background that don't require an internet connection but these are not possible with the current scheme..