Not sure of the cause, but this little vibration at a stop while in drive is absolutely normal. It's strong enough that you can actually see the steering wheel vibrate a bit. I just bought my A4 a week ago and test drove several after the salesperson said, basically, "that's just the way it is." He was right--it was the same in every car we drove (2006-2008).
I also have a 2007 Passat wagon with the same engine, but no vibration whatsoever. It's a bigger car, though, not as sporty in terms of suspension and the engine is mounted differently.