This is the situation I am in with 2012 Wrangler on order. From my experience, I was able to get two local dealers to guarantee me a trade-in price on my car for when my Jeep was expected to be delivered. Shouldn't be that tough. Just make sure both parties are in agreement about the miles you plan to put on the current vehicle before December. Also take into account (at least for yourself) how much you think the car will be worth in 2-3 months. If you're still paying on it, remember that you'll owe less in a couple months, thereby decreasing the payoff.
I tried that when I first ordered and couldn't get my dealer to give me a real appraisal..claiming they couldn't guarantee the worth a month from then. ...I ended up listing it for Craigslist for about $1600 more than the "trade value" listed on KBB and Edmunds. I wanted to get rid of the payment quick and found that usually the dealer's "trade value is a bit lower than those sites anyway.
When I ordered a 10 Rubi I traded an 06 TJ. There was no problem for me with the dealer. The only thing they did was look at it quick and since it had no body damage everything went smooth. It's up to the individual dealer how it's going to be worked out.
I made them guarantee the price. I currently drive a Honda minivan and it seems those are in high demand due to the problems in Japan. I was told the price would probably go down by the time my Jeep actually arrives. We worked out a price that will not go down. It was about $1300 higher than I expected to get as a trade in - I had done my homework. If minivans are still high demand vehicles and the price goes up, then I was told I can get the higher price. I think they know I will check it since I went into the negotiations with a spreadsheet of the options I wanted and prices!
I'm in the exact same situation and I talked to the dealer at length about it today. The purchase order has the trade value on there. I said "what about the miles" and he said "try to keep it around 1,000 miles or below". I said hypothetically what if I put more miles and the appraiser changes the value at the last minute and he said I could back out of the new car at any time and get back my deposit. So basically my plan is to avoid using it as much as possible and keep it in the garage as close to 100% of the time as I can and only use it when I absolutely need to.
I did comment that it seems funny to limit me to 1,000 milles when the truck I'm trading in has close to 128,000 miles. I said if the truck has 129,000 miles or 130,000 miles does it really make any difference??? Oh well.