Sounds like the same issue I had on my GTS. My front discs were replaced under warranty.
They were warped. I had to challenge their original position (which was to charge me for the replacements) but the dealer came thru.
All is good now, no more vibration.

 

Thanks, Zieg - I'm assuming it's warped rotors or I hit a pothole and need an alignment. Car goes in next week to determine the issue. It sounds like many here aren't too happy w/ the OEM rotors so maybe I'll look for aftermarket.