[JKT]

You can continue listening to your own music while playing games on an iPhone, so I don't think this would be an issue for third-party browsers, and as far as I am aware, mobile Safari doesn't keep running in the background when a phone call arrives; it actually quits but is re-launched again when the call is over... even so, other apps have this capability too, though I am not sure about phone calls e.g. NetNewsWire will jump back to its own view after looking at some QT film in the media player.

Btw, you mean multi-tasking, not threading - I think all the apps on the iPhone can be multi-threaded, they just aren't able to multi-task with other apps in the background, with the exception of those few Apple apps.