According to theBigBoss.org the developer Crash-X created an app that tricks the iPhone into thinking it is on a WiFi connection when it is really on 3G!
BigBoss states that there are two reasons this app would be useful:
1) You can install appstore apps larger than 10mb over the 3g
2) You can use Voice over IP apps such as Fring on 3G! (Think free phone calls, no minutes, everywhere you have 3G).
From the developer: This MobileSubstrate library can trick any application into thinking that it is on Wifi even though it is on a cellular data connection. There are a lot of possible uses for that, but for me the main use is being able to use VoIP programs like Fring over 3G. You can configure what apps get tricked by adding the app’s bundle identifier to “/Library/MobileSubstrate/DynamicLibraries/VoIPover3G.plist”. Currently, the default is to trick only appstore and fring. If you remove or rename the VoIPover3G.plist ALL applications will be tricked. This is not recommended though, because for a lot of apps its makes sense to know whether they are on wifi or 3G.
If you use it for VoIP, it might be that your internet provider blocks the ports that are needed for the application to work. This cannot be circumvented by this library.
Note: If your provider forbids VoIP and catches you and you have to pay for it, I am not responsible!
*Currently the app only loads/is supported by Fring, App Store, iTunes Store, and Truphone by default.