If you are building a system that needs to dynamically make decisions about items around it have a look at this Vision Object Detection and Ranging project. I think making this would be an excellent start to building your own Sentry gun!

"This particular setup consists of 2 X 3Com Homeconnect Webcams I bought off of Ebay about 4 years ago. They are fairly good cameras for the price ($12 to $15) and more importantly they have Linux drivers readily available. The computer is a donated Intel Celeron with a 2.60GHz CPU. The OS is Fedora Core 8 with a variety of other fun free software packages. Everything is shifted red in the images because I had to remove an internal cloudy blue filter from inside the cameras. Previously the cameras were extremely blurry. The software should be able to handle any webcam as long as it has a Linux driver."