First off, I think iAlertU is already great!
I just thought of a few improvements points.
Maybe an idea is to use a gif-image as a lock screen. This way the screen can be saved (burn-in's) and still the lock message can be used instead of a black screen.
And if the alarm is triggered, not just flash the screen but also the option to use another jpg- or gif-image. The user can make it as flashy as they want themselves.
Also it would be great if the alarm sounds could also be turned off. Like add a feature in the menu: "Default Alarm Sound ... Custom Alarm Sound ... No Alarm Sound".
It would also be great if arm and disarming could also be controlled by applescript. Then I could arm my system when I walk away with my bluetooth phone and disarm it when it's within reach again.
Maybe I'm the only one with these requests, but it would be greatly appreciated. If any help is needed, let me know!