It also freezes for me. It also happens when I leave the emulator open and do not touch any buttons for a period of time - the screen goes black and the whole OS freezes. It's a bit annoying, but manageable.
I have noticed this. It also does this when I am using a bluetooth controller and TV-out. This means I need to move a mouse every few minutes to prevent the screen from turning off.
However, this does not freeze the Pandora for me. It only turns off the screen and the emulator begins running at a stuttering/snail pace with skipping audio. After pressing any button on the actual Pandora or moving a mouse, the screen turns back on and the emulator goes back to normal.
I believe ptitSeb said the stuttering/freezing has to do with the SGX driver not able to handle turning off the LCD. Perhaps this also freezes the unit depending on your model and SGX driver.
I would really love a way to stop the screen from going to sleep, but I can't figure out how to stop it. This also happens with Love2D games if the Pandora nubs are in joystick mode and no other buttons are pressed for a while. I noticed this as the Love game I am working on is currently limited to simply being a twin stick shooter until I can add more features, so I rarely hit other buttons. It seems that joystick inputs do not count as real input for some reason, so the screen goes to sleep if no other key or mouse input is registered...
EDIT: I do need to mention that this is a VERY minor annoyance. It is easy enough to work around, and will be a moot point for my game once it gets more features. I appreciate ptitSeb's emulator and Love ports way too much for me to truly complain. That is the reason why I haven't been asking ptitSeb to try to fix it for me, even though I think about asking every now and again...