If it does, the reduced lifespan is probably far beyond the life of a PC component when you consider upgrades etc. Not heard of it being a problem myself.
Only reason it got me thinking is because very solemnly do I hear a server motherboard or CPU dieing. I've witnessed myself one or two Socket 7 boards dieing but mind you doubtfully these boards were in-use up to the point until I purchased them. Now that I think if it however it might actually expand the life of the components.
Back to the example of the light bulb... if you put in an eco-bulb rated at the same UV rays (not watts) it outlives a standard bulb in normal on/off operations. In fact a normal bulb use to last me about 6 months... these eco-bulbs I still haven't replaced one of them yet and it's been roughly a year and half. So in short a computer going into S3 mode will indeed lower its lifespan but it being in off state will also slow down it's life expectancy giving it more time than expected. Let's just hope -- on/off s3 state =/= continually on -- and balance out to be the same to make it worth the $12/yr power savings.. woot!