I wrote to several practices but had virtually no luck. At best I would get a response saying "maybe in the future, but not right now." I'm waiting on a response from one school...if it's an acceptance then I won't feel too much pressure to get the "shadow" notch on my belt. If I'm rejected I'll start cold-calling surgeon's offices. I think surgery is a little better than a PCP practice because a) there's more "doctoring stuff" to see and b) it's easier to schedule in your day (for those of us who are rather busy...). If you shadow a PCP you'll have to volunteer on some kind of schedule for some duration of time. At least with surgery you can maximize your time a little better. Also, if you can find a practice that has their own operating facilities, it will be easier than a surgeon who uses a hospital. Even so, my local hospital just requires a form which I have on hand to provide any doctor with; it requires the doctor's approval, the patent's approval, and the department head nurse's approval. If the doctor is willing to let you shadow, they should be willing to ask the patient. After that you just have to convince a nurse to let you observe (and with the approval of the other two, this shouldn't be a problem).
The downside to this is that a lot of schools would prefer PCP shadowing because you get more of the doctor-patient experience. Still, take what you can get! I've never asked my PCP because I think it would just be awkward, but if I'm not accepted this year I'm willing to pull out all the stops.