If it does happen, it's going to be because Intel manages to actually deliver a good competitor to the current ARM SoCs found in tablets and phones. I don't expect that to happen for at least another two years, but Intel has to be worried that phones and tablets may be the primary computing devices of the future so they will definitely try to break into this space.
Depending on how well Llano and Bulldozer turn out, AMD may either end up gaining a decent chunk of market share, or they might end up dead in the water. Either way, I could see ARM deciding to pounce and buy AMD, or at least be the dominant partner in any merger. It's going to continue to be much harder for AMD to hold strong against Intel and eventually, Intel is going to make a decent chip for tablets, etc.
The only way I don't see these two ending up together at some point, is if some company like Apple or HP were to snatch them up first. Of course as time goes on, that becomes increasingly unlikely as viable competitors in that space would grow. If tomorrow HP or Apple were to buy ARM and lock other competitors out of the next ARM SoC, it would cause a lot of chaos and commotion; but three years down the road, not so much.