Okay Jim, how about Toyota!

Back when I was active in repair we were strongly warned against mixing the Dex Cool type product with any other chemistry, most especially the old standard ethylene glycol. I presume you knew that, but worth mentioning for discussion sake.

There's lots of "stuff" on the internet about this issue, and it seems worse in the 4.3 than in other applications, though not exclusive to it. Most commentary on the net seems aimed toward sueing GM in the end, and is short on analysis of cause, long on opinion that sounds fishy to me. Here's an article that deals with the chemistry and possible causes that are more plausible. Seems that hard water, air in the system, and operating with low coolant levels are repeat offenders; http://www.babcox.com/editorial/tr/tr110046.htm
See commentary about half way down page. Babcox publishes a number of magazines aimed at the automotive service industry.