An analysis of the claims of Apple and leaked benchmarks with regards to the graphics performance and technologies in the new A8 chip that powers the iPhone 6 and iPhone 6 Plus indicates that Apple has likely moved to a new PowerVR GPU. The most likely candidate is the six-core PowerVR Series6XT GX6650 made by Imagination. Apple is also likely using a revised version of the Cyclone architecture, first used in the A7, to manage graphics processing.
The analysis was conducted by Anandtech, which says that only the GX6650 or a custom version thereof meets all the claims Apple has issued about the graphics system thus far -- that it is 84 times faster than the original iPhone's graphics system, that it supports OpenGL's Adaptive Scalable Texture Compression (ASTC) -- and is power-efficient enough to run at the same speed as the main processor, at 1.4GHz.
While the speed is only a small improvement over the A7, the main improvements are likely to come from what the site calls "an enhanced or otherwise significantly-optimized" core processor in the A8, which it cites as an impressive accomplishment over just the course of a single year. The site has not yet been able to obtain an iPhone 6 or 6 Plus for teardown purposes, as they will not launch until September 19. A teardown would confirm its theory, but the GX6650 fits the requirements of the new iPhone, and Apple and Imagination have an ongoing and strong relationship, making it an almost-certain likelihood.
The new OpenGL ASTC standard could be a key factor in the improved graphics and power efficiency of the new iPhones. It allows the PowerVR chip to be more efficient while still compressing nearly any texture to match performance needs. The standard is very new, however, and it will take time for game developers to widely adopt it.