The Record Screen will not allow your iPhone to auto lock or auto sleep. We force the iPhone to keep its screen on so that, in long recordings, the user can confirm that Voxie is still recording, by looking down at the phone. There are different things that can force Voxie to stop recording beyond its control (such as receiving a text or phone call, etc.). So, you should put your iPhone in Airplane Mode if you will be making a long and important recording that you don't want to be disturbed.
The Express screen, however, does not have this feature and auto-lock will happen. This is because Express is designed for short recordings only.
If you tap the Power button yourself on any screen, Voxie will stop recording.
However, in versions 2.0 and higher (2.0 should be released sometime in March 2009 - it is awaiting Apple approval) we have changed this behavior. Voxie will not sleep on Express Mode screen. And, if you tap the power button, Voxie will continue to record. But we are concerned that someone will sleep the screen, Voxie will continue to record, but a text will come in and the user won't know about it, and Voxie will stop recording (not under our control) and the user would never know. So, please be careful to check every so often if you are recording something important.