1) Setup quicktime broadcaster (it's free http://www.apple.com/quicktime/broadcaster/
2) Set you iMac monitor to go to sleep, but make sure the computer is set to never go to sleep.
3) Watch the stream on your PB in the other room.
If lighting is an issue, be aware that most webcams and camcorders can very easily pick up infrared light. For a few dollars, you can pick up an IR light source at the local hobby shop, and it should give you enough light to see what is going on in the room ( to see how this might work, point your remote control at your iSight, and press a button to see it in action . It works great on my camcorder. Haven't tried it with the iSIght though)
Alternatively, just get an IP webcam.
They're about $150. Check out D-link.