The iPhone 3G's most annoying feature

The problem is the iPhone's tendency to switch from the iPhone's built in wifi connection to the potentially expensive mobile phone network whenever the phone goes to sleep mode.

When you're connected to the Internet using wifi, the iPhone overrides GPRS/EDGE/HSDPA, but when the phone goes to sleep it shuts down wifi to save power and uses the mobile phone network to monitor your email and run other background processes - racking up your data bill.

On an unhacked iPhone or iPhone 3G there is no way to disable this, but this isn't a problem in the US because the phones link to AT&T unlimited mobile data plans. In Australia, this is a huge problem.

There is a fix, but it only works on older 1st-generation iPhones. On an original iPhone the easiest mobile data workaround is to dip into the menus and change your APN to gibberish when you don't want to access the mobile data network.

On an iPhone 3G it is not possible to edit the APN, unless the phone has been unlocked by your carrier or your carrier enables editing the APN in the menus. You can disable 3G, but this drops you back to 2G speeds rather than disabling mobile data completely.

Insomnia is a third-party native app for hacked iPhones that keeps the wifi link up when your phone is sleeping, but this chews through the battery and it still doesn't block mobile data access when you're out on the road.

There are a few elegant solutions to add an "EDGE off" switch to the menus of a hacked iPhone - third-party native apps such as services.app, iToggle and BossPrefs - and we got the best results from BossPrefs on a 1st gen iPhone.

These apps also let you disable SSH, installed by the hacking process, which should improve security and battery life. Similar apps will likely appear for the iPhone 3G, although they may not get Apple's blessing.