If it was "normal" we would have heard about it by now.
I have a 09 CLS550 and had the driveshaft replaced at 105k miles. I would get a vibration at 65 mph, just holding speed. The vibration was not noticeable on acceleration.
I felt strongly that it was the driveshaft, having been there before, but Mercedes wanted to try many things to solve it before they would admit that the driveshaft could be the culprit. They straightened 1 wheel, checked all brake calipers for sticking and "de-tensioned" the exhaust system (which I felt was a ghost). This is where I stopped them and went to my local independent mechanic. He had no doubt it was the driveshaft. He replaced it and there has been no vibration since.
Does your car has a 7 speed transmission? I have heard a few complaints that, when cold (New England winter, 15 degrees, type cold) it will cause a vibration on acceleration, until the fluid warms up.
Do you have a trusted non Mercedes professional mechanic?