I think the two biggest things standing in the way of a company like AMD putting an enthusiast class GPU and CPU on the same die is TDP and market reception.
For the first, imagine a single die that requires 350W or more of power. Current CPU heatsink/fan combos, even the fancy aftermarket priceir models, are really only designed to dissipate up to about 150W, if that. We are seeing GPU air coolers that are managing 250W+, but these are becoming quite large and noisy when pushed to having to handle anything above 150-200W (ie, at load). As a result, a combo enthusiast die would require quite an obnoxious air cooler in all likelihood (especially if one wanted to overclock).
The added cost in motherboard design to make something that could handle a significantly higher power envelope, higher memory bandwidth package might be another factor AMD doesn't want to risk - and even if feasible, may well drive up the cost of the requisite motherboard compared to currently offered alternatives with separated enthusiast CPU/GPUS, making it a less desirable alternative for customers.
I think the most important reason is market reception. AMD is pretty much the only company that could offer such a product right now. Yet they don't have the best reputation for high end CPUs at the moment. This is the great AMD catch-22. With Trinity, AMD is still playing it relatively safe and following Intel by adding a GPU that doesn't take too much extra resources than a CPU alone, both in cost to AMD and in memory/power requirements. As such, they can still offer Trinity at affordable prices (competitive with Intel's chips). Sure, A few enthusiast may be OK with paying $400 or more for an AMD CPU with an enthusiast class GPU (ie, HD 78XX o greater), but perhaps AMD is just too wary of market reception as a whole of such a product. In other words, perhaps they're afraid it will just rub people the wrong way to pay so much for a CPU that is outperformed by an Intel CPU that is half the cost, regardless of the GPU performance. If AMD's attempt to manipulate the media coverage of Trinity to be more favorable toward the GPU performance side (by encouraging early coverage of GPU performance) is any indication, I think this is likely.
Ironically, if Intel was in the same position GPU-wise, I wouldn't be surprised if such a product was in the works already.