The reasons cited are the weight of the glass, its tendency to scratch and crack easily, as well as the enormous difficulties Apple encountered in painting it reliably in white. Another change in store for the next iPhone is said to be the antenna steel band surrounding the current iPhone 4, which might be moved back inside now, considering all the issues Cupertino had with the "death grip" and reception.
All valid points, although it will be a shame to bail on such a unique steel-and-glass design. Furthermore, considering the number of rumors about both a cheaper iPhone, and one with a larger screen, it is not entirely clear which one the source is referring to.
The Japanese Mac blog that spread the word links to a supply chain analysis they have cited before, which refers to the next iPhone, but it's not the one that mentions the back panel swap and antenna redesign, so the original source is still to be excavated, if you know Chinese. The Japanese from Macotakara have also sketched up a pretty weak mock-up of what such an iPhone might look like.
What do you think, just another unsubstantiated rumor, wishful thinking, or laws of physics?
source: EDN (translated) & Macotakara via MacRumors