Thanks for all the suggestions. I got it out today after work. I decided to try drilling it off center to release any tension on the threads. I then changed my mind and went for drilling the head off the bolt and dealing with it later. When I had the hole drilled, the head was still on the bolt, so I put a wrench on to snap the head off. Instead, the darned bolt backed right out! Nice as you could ask. Go figure!
Anyway, on to the next step, pulling the oil pan to check for sludge prior to putting things together.