Offhand, I think I've been upgraded in advance between LAX and ATL exactly once in the past 3 years (out of 30+ legs) -- that was on a 777. All the other times it has gone to the gate. Mileage and Elite upgrades do come out of different buckets, but since (as others have said) mileage upgrades don't go the gate, I think you'd have a poor shot. Discount FC seems to run about $500 over the cheapest coach fare so that might be your best bet if you have to sit up front.