How long should roast pork be cooked so that it stays light pink?
The important part of cooking meat to a safe range is a combination of time and temperature. The ranges that you often see from people like the USDA are a temperature at which everything nasty dies instantly. This is a much more definitive test than saying that you need to reach a temperature and hold it for a certain period of time. The US government has a great table showing time and temperature for cooking pork safely (just scroll down a little from the link). As you can see, it's safe to cook meat well below the often given 160F. In fact, it's instantly safe to eat at 144F. I assume that the reason for the 16 degree buffer on the often stated temperature is just to make sure people don't run afoul of the magic number. 

What you must understand is that, at these temperatures, it's imperative that you don't mess about with the amount of time. They must be held for at least the time given. But, 140F only needs to be held there for a minute. Once you've got something all the way to 140F, residual cooking should hold it there for at least 60 seconds. Just don't play this one fast and loose. 

Cooking something like a roast is slightly more complicated than just getting the meat to temperature though. Generally there's a lot of fat and intramuscular tissue that you want to rend and denature respectively. That's what makes the meat so juicy and tender. While this will begin to happen at lower temperatures, it takes longer. So the meat may not come out as tender or flavor full as you'd like. 

I would suggest a little experimentation. As mentioned, the thermometer will be a huge help if you want to be able to do this consistently (due to varying sizes of meat). I'd throw it in the oven and run some tests. I usually do Boston Butt to 168F, and this comes out with some pink parts, but I'm not sure how pink you want the meat (or how much of it should be pink). I'd recommend starting at about 160F and then moving up or down in temperature based on how the meal comes out.