Beginning at approximately 75 miles per hour, the car begins to vibrate, as if the road were a bit bumpy. The vibration gets a bit worse as speed increases.

This is a brand new 2008 A6 with about 2500 miles so I doubt the wheels need to be aligned.

I have seen some other threads and posts about vibration, but they were either different than my problem or never reported any resolution. Any ideas?

 

Where do you feel the vibration? steering wheel? or underneath like in the seats?

i'd say it's still the tires (since your car is so new).... are they stock size? sometimes just being a size smaller on the width can cause this....

my A6 stock size is 235 45 17 and I had 225 45 17 on it when I bought it and it vibrated, then when changing my tires I realized the size difference and once the new ones were on there was no more vibration.

Also warped rotors can cause vibration as well.

 

Unless you'd like to skip the warranty => this is a dealers problem, not yours.

 

I just had the same problem....sorta. Highway virbation - changed all the tires, no luck. As it got worse, I figures I'd narrowed it down to front right. I was thinking C.V. shaft or bearing. I got home and smelled brakes burning. Left rear caliper was siezed on - after taking it apart there was nothing left of the piston dust boot, and the piston siezed to the housing with white powder (aluminum oxidizing).

To make a story short, after speeding up to highway speed, the rear rotor heated up and warped, causing the vibration to go through entire car. fixed the problem and its smooth sailing!