This was one of the first things I noticed when I picked up my "new" used Focus a couple of weeks ago (I never noticed it on the test drive though). I would be sitting at the lights with my foot on the brake and the car would be vibrating during idle.
I took my car in to a Ford dealership this past Wednesday and told them about it and they fixed it. The previous owner of the car would have received (so I'm told) notification for 2 recalls, one being the sync and the second being reprogramming the PCM, TCM and ABS module. This second recall is what fixed the shaking car during idle and it in fact did fix mine, recall 12B37B for your reference. It may fix your issue, it may not but it's worth a shot.