as another poster remarked, you cannot use the spare as a rolling tire and have the system tell you that it is going flat. that does not mean that you can't use it in a 5-wheel rotation, you just have to check the condition and pressure of your tires regularly. if you have a sudden deflation due to severe tire damage, you'll know about it.
I did a little research about this topic, schrader electronics is the vendor for the transmitters and receiver for the system. apparently they have been supplying GM and others for a few years now. I found a DIY page for corvette owners on how to reset the system. using some kind of magnet tool, the transmitters are 'forced' to send a signal out.
anyway, I e-mailed schrader and they told me that you could by a 5th sensor for the spare, you would just have to retrain it at each rotation so it would know which 4 codes to look for on the rolling tires.
so, question of the day is, can this be done by the owner? I think so, it just will take a little more digging. :computern