"Currently, we expect [World Tour] unit sales to decline by more than 50 percent series-over-series for November. This is coming off the October month where series-over-series units declined by more than 60 percent," said Divinich today in a research note, according to Edge.

Divinich points to the below-retail prices of World Tour on Ebay as an idication that "demand and supply are currently in equilibrium."

Though Divinich suspects the series has topped out, he notes that this doesn't mean Guitar Hero will fade away--only that the rapid growth of the franchise has burned out.

"We expect Guitar Hero and Rock Band releases for the next 10 years as they will always have a large and loyal market base, just as [Dance Dance Revolution] is still today a very profitable franchise for Konami, even though that series reached its peak a long time ago," he added.