Android OS2.2 used to have an option under Settings/Applications/Development to disable screen lock during USB debugging. After upgrading my Samsung Galaxy S to OS2.3.3 this
Two options come into my mind:
write an app which will force screen timeout to be very high. Use SCREEN_OFF_TIMEOUT or STAY_ON_WHILE_PLUGGED_IN.
If your phone is rooted and you are connected to wifi in the same network as the computer you're developing on, you can enjoy this wonderful app which comes with an option for screen timeout too: wifi adb.