Filesystem:/Applications/FieldTest.app/FieldTest

This is the actual executable for Field Test.

iOS 4.x Workaround
Field Test is not enabled by default in iOS 4.1.

The Workaround is to play with DMG file of iOS 3.1.2 of the same device
 * 1) extract Field Test app and copy to your jailbroken iOS 4.1
 * 2) copy FieldTest.app from var/stash/Applications of 3.1.2 to var/stash/Applications of iOS4
 * 3) copy fieldtest.plist, fieldtest68.plist and specialkeys.plist from /System/Library/PrivateFrameworks/CoreTelephony.framwork/Support to /System/Library/Frameworks/CoreTelephony.framwork/Support of the iDevice

In this way you will have full access to the Field Test app along with all possible options.