At the top of the screen, on the right side of the carrier name, you might see a square or an "E". If you see a square, it means there is GPRS coverage. If you see an "E", it means there is EDGE coverage. In both cases, when the icon is blue instead of white (in the standard theme) it means there's an active data exchange - according to your data plan this might be sucking money or not.

Provided that your SIM has a data plan enabled, you can use EDGE to connect to the Internet when a wi-fi connection is not available.

Go to Settings / General / Network / EDGE, and provide the APN, username and password settings. You should be able to retrieve these settings from your carrier's website, or through their tech support.

When you launch Safari or other applications that need an Internet connection, the iPhone checks if the wi-fi is on and if it can connect to a network; if it can't, it automatically uses EDGE. If you don't have a flat data plan on your SIM card, this could easily consume all your credit!

To permanently disable EDGE, go to Settings / General / Network / EDGE, and type a wrong value for the APN setting; this way the iPhone won't be able to connect to a valid EDGE connection.

If you want to disable EDGE just temporarily, and be able to easily re-enable it at any time, download Boss Prefs through the Installer application, then launch the new application and use the switch to enable/disable it.