I let my car warm up year round, at least until the revs fall to normal (900 rpm). I`ve done this since day one and my car is six years old, has 65,000 miles and has never failed to start nor has it left me stranded somewhere. The car still has the original battery in it and the spark plugs have never been pulled for any reason, not even when I installed the Jackson SC almost three years ago.
I think that the reason auto makers don`t recomend warming up a car is because it uses more fuel and thus hurts their gas mileage figures. End of story.
2003 ZX5 SVT, Pitch Black, JRSC/BBK Sold