I do one single shoulder press on the Smith machine two or three times a week, pretty much regardless of what weight was left sitting on it. The Smith machine is right next to the squat rack so, rather than ducking under and around the bar, I lift the Smith bar up out of irritation's way.
It doesn't seem to have done any harm at all to my delts. Great machine.
Interesting experiment, madcow. I'm occasionally tempted to return to the gym I used to work out at and see what difference to my Smith bench free weights have made. When I left that gym in March, my Smith bench was at 135Kg for a triple. I never quite managed to press three plates but I quit the gym the week I got the triple. I've since pressed 3 plates on free weights after about 6 months work. There's a chance I'll be doing some work near there again in which case, since they have a great cardio selection, I'll probably join that gym for a convenient lunch-time workout. If I do, I'll report back.