Dexron-VI is fully compatible for all prior applications calling for Dexron-III(H) and older. There's no hesistation on GM's part with regards to claiming this; they have been very clear upon the initial release of this product to advertise its backwards compatibility in all Dexron applications.
Apparently, licensed Dexron-III(H) will continue to be avaliable until 12/31/06, which is when all licensing will cease, apparently. I just picked up two gallons of SuperTech Dexron-III(H)/Mercon today at Walmart for $14 total.