I was thinking about this topic this morning on my way to the office. I normally drive 25 miles to work about twice a week and half of it is 65MPH zones. I was coming up US1 north and gunned it a little to get around a couple of cages that were doing dumb things (mostly crossing the white lines like they couldn't see them). Anyway, I digress....
Now, I'm 200 lbs and only 5 ft 6 but I got it up to somewhere between 75 and 80 and noticed it vibrating significantly more than usual for highway driving. But it didn't feel unsafe, just irritating.
I know after 10-20 minutes of that type of jarring, I'd be ready to take a long break with some Pepsi and peanuts at the Sheetz somewhere nearby. That's a southern thing, pour a package of peanuts into a bottle of Pepsi and kick back a big 'ol swig...... Mmmmmm. But again, I digress.
Like J.O.G. said, it's also quite relative. I think a bike that is less than 400 lbs is going to shake a little at that speed. But definitely be sure your tires have decent tread and are aligned well.