The tweak is called LockLight, and is in the ModMyI repository in Cydia.  Searching for LockLight should bring up the application.

LockLight uses the Cydget platform, which allows for multiple lock screen views. Cydget is configurable in Settings.

Configure Cydget such that AwayView is first, and LockLight is lower down in the list.  Unfortunately this tweak works best with just LockLight and AwayView, but there are other cool Cydget views that are worth looking at, such as CyCalc.

Once you’ve configured Cydget, Respring your device and enjoy using LockLight!

LockLight Activation:

To start using LockLight, press the power or home button to view the lock screen.  Tap the home button to cycle through your enabled Cydgets.  If just LockLight and AwayView are enabled, tapping the home button will alternate between the two.

I’ve realized that the issue is that pwnage tool will automatically select “Activate” when Jailbreaking the firmware image.

In order to use your iPhone with OS 3.1 and AT&T, remember not to select Activate when jailbreaking.

Select General to modify Activation options.

Remember to unselect Activate the Phone.  This will allow the iPhone to work on AT&T with the proper activation.