A pictorial and a video demonstrates how to build an invisible switch into a PC. A schematic is shown how to apply this technique for cars.
Step 1: How to make an invisible switch in a PC step 1
The first picture shows the reed switch which we will use. It's a glass tube with two contacts which are pulled towards each other making contact when a magnetic field is present. You can reed(read ;-) ) more details on wikipedia. http://en.wikipedia.org/wiki/Reed_switch
For the PC we'll use a read switch with a make contact.
The first step is to remove both side pannels from the pc housing by unscrewing the bolts shown in the picture. After that the front panel can be removed by pushing the front panel retainers together with your fingers to unlock them.