Here is the description that I put with the app on the Android Market:
3G Connection Fix is a simple tool to automatically work around a known problem on some phones where automatic switching from Wifi to 3G/packet data does not always work.
Important: This app will only be effective if the manual (Flight mode) workaround described below successfully activates 3G data.
The simplest (manual) workaround for this issue is to turn on "Flight mode" and then turn it back off again when you notice no 3G data connection. This manual workaround is annoying, and if you forget to do it then you are without a data connection.
3G Connection Fix will attempt to activate 3G data by automatically toggling Flight mode on then off when the phone disconnects from Wifi, if the phone doesn't switch to 3G by itself.
This app has only been tested on a Samsung Galaxy Ace, but may work on others. If you try this on a different phone and it doesn't work please email me. If it works for you please leave a comment (or send me an email) stating the model of your phone.
1. There is a 1 minute delay between your phone disconnecting from Wifi and this app attempting to activate 3G data to allow the phone time to try to switch to 3G by itself.
2. If you are on a phone call this app won't disconnect you but also won't attempt to activate 3G data.
If you find any bugs please email me rather than leaving a comment on the Market (as I can't reply).
This app was written to work around Android Issue #2207.