Clarifications: all 5 tires have a TPMS sensor in them, but as far as I can tell they the computer doesn't/can't read the one in the spare tire position. Mine had the TMPS recall done around the time of the Autoblog post. It takes 2 minutes at a dealership, so anytime your PM is at a dealer they probably did the update and may not have even told you. Maybe it's germain to me, but my spare currently in position and totally flat, and I don't have a light. Another point - the computer is pretty smart. It knows what the pressure is supposed to be in each position (65 front, 72 rear), and will learn each tire's position in a mile or two. From past experience with a leaky tire, I'd guess the threshold to light the light is 5psi, maybe as much as 7psi.
Regarding not carrying a spare, I could see both sides. About 1 year ago I got a puncture in the PM, and was darn glad to have the spare even though it was an adventure learning how the winch/jack/iron worked at night in the rain. But it made the difference between delivering on time and not delivering on time. But for local travel I see the appeal. I most of my passenger cars, I ditched the spare years ago because the x-wife was pathologically unable to learn how to change a tire. So what's the point. If you, or whomever is driving isn't willing and able to change it, then there's probably no point in carrying one. TravelDerby makes a very good point; tires don't go flat as often as they once did.