I wonder what would be considered inactivity? It seems like better control is needed over the packet network connection for sure.

But I have IM, RSS, email, weather, sip all wanting to access the internet, and so triggering "activity".

None of these require 3g most of the time. The ideal management tool would allow you to define the networks that each application could run on.

Wifi only, wifi+gsm, wifi+3g. This doesn't seem likely to happen so I would settle for a manual switch between packer data off, packet data gsm only, and packet data 3g (from an icon on the desktop or via status).

Can this be changed from the command line - perhaps via dbus?