Interesting, I took home my STS 2 weeks ago and found the same vibration above 60, read up on this known issue and took it back to the dealer and told them to balance the tires. Got them back, they were balances out to no more than 1/2 oz on each tire, took it out for a test, it still vibrated. Went back to dealer and told them to get it as close to zero as possible. And take off the lock nuts on the wheels (they break sometimes, when removing the wheel). Got them back with close to zero as possible on each tire.
No vibration at any speed... I was happy, $125 well spent ($75 for the speeding ticket(cop gave me a break from 50 over the limit to 20 over), $50 for the balance job).
This reinforces what I have read, that caddy's are very sensitive to tire or wheel issues.