The problem with synthetics in a new vehicle is that they work SO well that they will not let the engine wear and break in. I would say wait at least 10,000 miles before switching. I have heard of people getting better mileage up to the 10K mile mark. That tells me that it takes a car about that long to break in properly.
However, I run non-synthetic in my 2003 Cavalier. According to the owners manual I have to change the oil every 5000 miles otherwise I void the warranty. After my 100,000 mile warranty expires, that will be very soon, I will use Mobil 1 and change the oil every 10-12K miles.