For the PlayStation 3, when it came to the console's graphics, Sony partnered with Nvidia to produce the RSX 'Reality Synthesizer' GPU. According to a report on Forbes, though, for the PlayStation 4, Sony is partnering with Nvidia's arch-rival AMD instead.
Citing "former AMD employees", the Forbes report says "the processor designer may play a key role in the new product". And...that's it.
Not the most extensive of rumours, then, but one that's significant enough for the mention of AMD, which already runs gaming hardware like Microsoft's Xbox 360.