L.L.
With my first, I measured big the whole pregnancy, and so did my daughter. They tested me for gestational diabetes 4 times (all negative). At birth she was eight pounds, 19 inches long and her head circumfrance was much smaller than what they measured in utero. I am actually going through the opposite experience right now - baby is measuring VERY small (I am due on 1/7/11). All in all I think you need to remember that ultrasound is a great diagnostic tool, but it is not 100% accurate. Because of the angles - location and orientation of baby - of the sound waves boucing back, you can have scewed measurements really easily. This is no fault of your doctor/ultrasound technician - it is just an aspect of this diagnostic tool. The same thing can happen with x-ray.
I would stay positive and talk to your doctor about your options. My friend who is preparing for a vbac said her doctor is fine with her going as late as 41 weeks even though her baby is measuring large, and her c-section was only 2 years ago. Each doctor is going to have a different comfort level with VBAC - some docs are very encouraging and some are quick to bail and cut.
Good luck!!!