I don't know why AMD would use a older architecture. Maybe it was because the APU and GPU developments aren't synced and/or the APU team has little time by the time the new GPUs arrive.Moreover, Trinity employs a newer graphics architecture than Llano. Instead of the VLIW5 arrangement, which also sat at the heart of Radeon HD 6800 and older GPUs, it utilizes the VLIW4 design that went into AMD’s Radeon HD 6900-series cards. Everything after the 6900s swapped over to Graphics Core Next, so VLIW4 isn’t a very prolific implementation. But it’s supposed to be more efficient. Naturally, then, we all want to see how Trinity’s on-die GPU compares to what came before.
What we know for a fact is that Trinity – the 2012 Bulldozer APU – will not use GCN, it will be based on Cayman’s VLIW4 architecture.