Maybe "acid" wasn't the best choice of words, but when you mix additive packages and rust inhibitors, you never know how they are going to react. After all, Dexcool is made with "organic acid technology." (http://www.sancarlosradiator.com/antifreeze_coolant.htm) Also, "the OAT (organic acid technology) will cause precipitation of silicates in the green type and corrosion protection is greatly reduced." (http://www.geocities.com/dtmcbride/home_garden/auto/antifreeze.html) So this probably won't make your coolant "turn into acid," but it will make your coolant more acidic than it was.
"If the coolants become mixed with Dexcool?, however, one study showed a possible aluminum corrosion problem in certain situations." (http://autorepair.about.com/cs/coolingsystem/l/aa052601c.htm)
According to GM, "Mixing a ?green? coolant with DEX-COOL reduces the batch?s change interval to 2 years or 30,000 miles, but will otherwise cause no damage to the engine. In order to change back to DEX-COOL however, the cooling system must first be thoroughly drained and flushed." (http://www.imcool.com/articles/antifreeze-coolant/dexcool-macs2001.php)
I don't believe it. I have seen too many stories about head gasket problems that only manifested themselves after the coolants were mixed. I also find instances where GM warns not to mix Dexcool with other coolants:
"On its website, the company now also warns owners to consult their vehicle 'owner's manual for the type of coolant right for your vehicle' and never 'mix one type of coolant with the other.'" (http://www.consumeraffairs.com/news04/2006/08/gm_dexcool.html)