What should happen when more low income people get health insurance? One possibility is that they will now go to doctors' offices during normal hours, so ER use will fall. The other possibility is that the increased insurance coverage leads to increased purchases of medical services across the board, including ER visits.
The answer is quite clear in the Oregon data: ER use increased by 40% for those newly covered by Medicaid compared to those who did not get coverage. So as Medicaid coverage expands in most states as the Affordable Care Act comes into effect, we should expect a lot more ER visits.
For further insight, be sure to check this post by Bloomberg columnist Megan McArdle.