It's pretty straightforward if you just open /Applications/Preferences.app/General.plist and see how it's done for other toggles. See my signature for ways to edit that file. However, if you need more information, see this thread by the guy who first found out how to do it, or this tutorial.

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iPhone 3GS / 3.1.2 JB (PwnageTool) / 04.26.08 carrier-locked, Carrier Logo Fixer / Cydia / 1000 posts on Hackint0sh

Installing Cydia programs on a phone that has no internet connection: read this.

Editing binary .plist, .strings, .nib and .xib files:
* on your computer: Windows tool / conversion website.
* on your iPhone: convert those files in a terminal with plutil (installed with Erica Utilities) or edit them with iFile (Cydia links).