Update:
Many of you have sent me PMs, so I thought I would provide an update.
I was very impressed with the response from Mazda Corporate. They sent two engineers from Japan to meet at my house and examine the vehicle. As previously mentioned, they also tested gasoline from several stations in my area. The engineers came on two separate trips and consulted engineers from Ford. The end result was an engine replacement and custom tuning so that the thermostat opens just before the start of going up the big hill. This was back in the summer of 2009.
Over two years later, and we have never had a problem with the new engine. I appreciated the support by Mazda Corporate and their response. I cannot say the same for my dealership, and I was glad Mazda had a mechanism in place that allowed me to talk to corporate directly without having to use our dealership as a go-between.
Hope this thread helps others and serves as a credit/testimony to Mazda for their customer support - thank you!
Last, as for my location, I live in Alabama, and BTW, my 1998 Mazda B4000 truck is still running strong with over 185,000 miles on it now.