it was my understanding that the tire pressure monitoring system in the '05 actually does monitor pressure... even in the spare... apparently using specialized valve stems to monitor pressure...
the option is the tire pressure monitoring display which shows the actual pressures in each tire in the EVIC...
i don't have my user manual handy right this minute, but i'm sure i recall it saying that the system monitors pressures, including the spare (how can it monitor the spare? it's not rotating) regardless of whether or not you have the EVIC display option...
anyway... mine doesn't have the optional pressure display function, but it definately has some heavy duty looking valve stems in each wheel so i'm sure that's how it works...
here's the next big question... can you transfer the valve stems to aftermarket wheels? hmmm...
mb818
edit: i just looked in my owner's manual and it indeed says that the base system has wireless sensors in each wheel's valve stem that signal a low pressure situation... the premium system includes the ablility to display the pressures on the EVIC... however, it's confusing as to whether or not the spare will trip the indicator light... it does say that a sensor is included in the spare if you hva a full size matching spare (don't they all?)... but the system apparently knows about the pressures in the four mounted wheels, but will still work properly if you follow a 5 tire rotaion schedule.. whew.. lotta stuff going on there.. wish i had a soft copy of my owner's manual.. would be easier to cut and paste 