I bought my PS3 two years ago.
Never had any problems with it.
My understanding is that the yellow light comes on when say... you've paused a game and left the room for a long period of time, or the same with a movie. Just leaving it on pause and walking away.
You can set it up so that it automatically turns off after X amount of time.
That'll help keep the machine from malfunctioning.
If I remember right the PS3 slim doesn't have a way of actually displaying a yellow light. So be default it wouldn't Yellow Light. Just like the newer 360's apparently can't red ring because the red lights are gone.
That doesn't mean the systems can't ever malfunction and stop working. It can happened to anything like that.
Never had problems with my PS3 Slim 250GB Edition (bought it in 2009), but I'd suggest not updating to the newest firmware update because it could brick your console make sure to do some research before so.