A friend of mine and I were just talking about this. He has a pacemaker that they put in that also has a recorder. The doctor pulls data off of the device using a wireless connection (so he doesn't have to open him up again). The device has no security on it (the doctor actually pointed that out at one point). Depending on the device you could theoretically kill someone by "hacking" into something like that (upload a new set of heart settings that drive you into a heart attack). The problem becomes though what happens if you loose the password (you are going to have to open him up to "reset" the device). Worse yet, what happens when you are rushed to the ER and they need to adjust your device. You may be unresponsive and not be able to give the password. Do they sit there with technical support for your device while you code on the table? I am not saying that medical devices don't need security, there just need to be a lot of checks and balances, especially when you are dealing with someones life.

This was the book that we used @ Auburn for our Analog design courses. Its a good book with a LOT of theory: Microelectronic Circuit Design