Lockdown is an amazing utlity for the iPhone v2.0 via Cydia. This app allows you to lock / password protect your applications using a Pin number. The best part is that this app allows you to lock all three; AppStrore applications, Cydia apps as well as the default integrated iPhone functions like Photos, iPod and Sms.
The App is free and installs via Cydia. Cheers
The upgraded version of Lockdown has some more security features, and that simply means that it keeps on asking you password again and again (somewhat irritating, but acceptable). And the only complain is no option to set different passwords for different apps … but we are sure it will come up soon!
Note: Before updating the app via Cydia, make sure you have unlocked all your apps. Else you might end up being permanently Locked out.
Cydia is an Installer alternative, that Installs on Jailbroken iPhone 2.x… Info here