UPDATE: I mean when the application has gone to background, the user closed it, iPhone is on the Springboard. Can be the connection kept alive?
I want to push some content to the clients. Is it possible? Do I have to register some... ...thing?
Just register the app as a VoIP app, then a socket will be mantained in the background.
Page 60, and Page 180: http://developer.apple.com/library/ios/documentation/iphone/conceptual/iphoneosprogrammingguide/iphoneappprogrammingguide.pdf
The actual background task can be subject of App rejection, you have to be careful.