I think checking the brake rotors for rot is a good idea, but on the other hand tires can cause a lot of wierd vibration problems. If your tires are old, I would go ahead and replace them before you drive yourself crazy chasing this vibration down - start with the obvious first! Secondly, I have read that it is very common for certain model BMWs to vibrate and highway speeds, and the ONLY way to eleminate the problem was to use a special balancing machine that balances the wheels while they are on the car. Ask around for a shop that can do this.
Hope you solve the problem.