Tom Yager, writes an article intended to instruct users on the eight steps to iPhone security.
The steps outline are:
1. Never leave your iPhone unlocked
2. Keep up with Apple firmware updates
3. Put your iPhone on a leash
4. Secure your iTunes host
5. Don't jailbreak your iPhone
6. Hide sensitive data in plain sight
7. Use FileVault on the Mac or EFS on Vista
8. Use Exchange Server for its back end
Paying attention to step five Yager writes,
The iPhone jailbreak process purposely disarms the mechanisms that Apple created to protect your data. With App Store, a trusted party tests and vouches for the software, and Apple can trigger an uninstall of an app if a risk is discovered later. The protections offered by open source projects --multiple contributors, readily viewable code, and a central location for comments and fixes -- don't exist in the jailbreak world. I'll grant that jailbreaking an iPod Touch or a retired iPhone can be good fun. Relying on a jailbroken iPhone as your primary mobile device is idiotic.
It's so quick and easy to jailbreak an iPhone that it takes a minimum of social engineering to trick a trusting user into bypassing Apple's built-in guard against modified firmware. It's a simple sell: By holding down one key while clicking Restore, you don't waste time waiting for new firmware to download from Apple. Don't fall for it. Always download firmware directly from Apple.
*Thanks to Chris for the link