As Twinkler explained to me:
In versions prior to OS 5.0, pressing the Mute key put the device into Standby mode, while pressing the Keylock key engaged the Keylock Function. Then came OS 5.0 … in some versions these original functions remain operational…. In other versions this function has been modified (we have confirmed this with RIM).
For example, in OS 126.96.36.1994 (Sprint Tour Official) – The Mute key no longer engages Standby mode. It has no function other than to mute the microphone while on a call. The new function is a Combination Standby/Keylock function. As such, all apps that put the device into Standby mode (including previous versions of AddOnis) could not work with the latest OS 5.0 versions.
With the latest version of AddOnis they have managed to figure out a way to put the device into this new “Hybrid” standby/lock mode. They have also built in backwards compatibility so users can specify which mode they want for their particular flavor of OS 5.0. There have been a few other bug fixes but you can get the latest 5.0 compatible version at this link in the store.