CodeGoo's iPhone application Baby Monitor is inexpensive and provides a way to monitor a room for sound, such as a baby's nursery. When the application detects noise, it silently dials the number you specify and allows whoever listens to monitor the room. It will also trigger if the iPhone is picked up. As the developer suggests, it can be used to monitor just about anything.
If you're looking for baby or child-related iPhone apps, see Great iPhone applications for Parents.