The solder joints on the PS3 motherboard are actually quite good (better than previous PlayStation platforms).
Because of Sony's bad rep for poor soldier joints, I would venture to say that its better to leave the PS3 on all the time. No thermal fatigue that way.
I don't think I'd try leaving a DVD or game in for a while, since I do worry that may wear out the laser.
Well, I had a friend who left his Dreamcast on all day, and it broke, which might be the reason I kind of fear a console breaking when leaving it on for a while, so that's why I don't leave a console on for very long.