I test drove two 328i's today at the BMW "Olympic" event. I felt the vibration in both cars. I told one of the staff about it after driving the first car. He blamed it on the lane departure warning system and showed me how to turn that off.
When I drove the second car I felt the vibration and turned off the system. I didn't feel the vibration again but I'm not convinced at all that turning off the LDS had anything to do with it.
Some people have mentioned unbalanced tires as a possible explanation but I seriously doubt it. The vibration I felt was almost like an electric buzz, and it came and went very quickly as if being turned on and off.
It may be a harmonic buzz coming from the engine or turbo and vibrating the steering rack or box. Obviously I'm not sure what is causing the vibration but out of balance tires have a different feeling ( I used to work in the wheel business).
Besides the weird buzzing in the wheel, I loved the car! I hope BMW gets this sorted out before I get my 2013 wagon!