The mobile operating system Android has a hidden service menu where some information about the phone can be viewed (Phone information, Battery information, Battery History and Usage statistics). To access this menu open the dialer and dial:
*#*#INFO#*#*
In numbers:
*#*#4636#*#*
I have tested this on my HTC Desire (Android 2.1 and 2.2) and in an emulator (Android 2.1). The following screen is shown:
Service menu
In Phone Information you can see some information about the phone, current network and data connection (modify settings or pressing buttons is at your own risk!):
Phone information
For more information about these settings and other menu items see this post.
Update August 15 2010:
In Android 2.2 the service menu is extended with Wifi information.