I'm not so sure that 'the poor, those with pre-existing conditions, etc' are increasing the demand for healthcare now that they have insurance. People always used healthcare, it's just prior to the ACA, the demographics you are talking about had to use it in the most expensive and inefficient way possible.
Studies have generally shown that increased use of preventative health care actually costs more, even in the long run. Which is not to say that it's a bad thing because there's more to human health than just cost (well-being is important too of course). But from the point of view of health care consumption, more preventative care increases consumption.
As for the issue of emergency room use (which is claimed to be inefficient and expensive), there was a very well done study in Oregon which showed that when people were given Medicare, their emergency room use actually went up.
That being said, I have always been skeptical of the claim that emergency room use is more expensive in terms of real resources. When you think about, it seems like a very efficient use of resources. Much of the use happens at night when most of the hospital facilities would otherwise be idle, the patients are queued up so that doctors waste no time waiting to see a patient (or even worse waiting for a no-show), and the doctors get to use a full range of their skills and training rather than only in the area in which they've specialized. Emergency rooms are bad for patients (who sometimes have to wait many hours to see a doctor), but I never understood why they should cost so much. I suspect that the high cost of ER treatment is to some degree an accounting fiction. Hospitals have an incentive to use the ER as a dumping ground for expenses because that's where most of the uncompensated care hits. Thus they can make it look like they're suffering more from uncompensated care than they really are.
Now that they have access to affordable insurance, I'd like to see some evidence that they will use more healthcare in the long run, rather than just spreading the pool of insured people, thereby causing costs to actually drop (or not rise as fast) for those who were previously insured.
You're adding new people to the roles, so aggregate demand will go up, even if average demand per capita goes down. In order to boost aggregate supply to meet this aggregate demand, cost has to go up. The only other way to balance things is if aggregate demand were to be curtailed through other forms of rationing (e.g. restricting networks or having long queues for services).
Another way of looking at it is that the vast majority of the new people being added to the roles are either implicitly or explicitly subsidized. Implicit subsidization comes in the form of higher costs for those who are not subsidized. Explicit subsidization comes from the government in the form of higher taxes.
Obamacare is supposed to try to dampen the high rate of health care cost growth overall, but it starts out making the task harder by subsidizing health care consumption. The move to higher out of pocket costs, as well as less flexible networks, will definitely help to control cost growth, but whether that is sufficient to dominate over the effect of increasing demand from subsidized people remains to be seen.
Perhaps the most effective cost reduction measure will be the Cadillac health care tax, but that doesn't kick in until 2018, and, frankly, I think it's unlikely it will ever kick in.