Shimmy can definitely occur when you get a bike where the rear triangle is not rigid enough for the amount of torque being placed upon it. There were several "touring" bikes that were under built in the 80's where this was a common problem if your load wasn't well balanced. But yes, I do think bike racks today in general are built stronger and with more rigid over sized tubing than in the past.

One last note, a close friend of mine bought a brand new Trek 520 two years ago and it shimmied terribly right from the get go. She is a small woman not weighing much over a hundred pounds and she was carrying only a moderate well balanced load with both front and rear panniers. Her frame size was really small so it should have been stiff as heck yet just riding along on flat ground, you could see that frame wobble in the rear. She took it back and no one could figure out what the source of the problem was but they replaced it with a Surly LHT and all was good. Not a knock on all 520's, but just shows that a poor frame can show up anywhere.