Froyo has a new class android.net.TrafficStats with the getUidRxBytes function (int uid) and getUidTxBytes (int uid) which allows for each application to receive the amount of sent / received traffic. The idea to write a simple program was born at once, it seems to me that the Google users meant it when they did this function. The program, when called, and once per hour updates the values in order not to lose data upon reboot, there is also an update button for paranoids.
A very useful and simple program that shows how much traffic various programs use. Shows which applications consume the most traffic.
Available only on Android 2.2, by the way, on which phones have the firmware already been released?