There is nothing wrong with mentioning that you are interested in working in an ER in your personal statement, I talked about wanting to be a pediatrician in mine. A good portion of my personal, voluteer, and work histories involve kids or teaching though, so I related that stuff to medicine through my interest in pediatrics (I also worked with a pediatrician and volunteered in a hospital on an orthopedic floor). When asked about it in interveiws I said I was strongly leaning towards pediatrics and definately most interested in primary care, but also mentioned that I haven't been exposed to everything yet and so I'm definately willing to reavaluate as I learn more about different specialties.
If you are really excited by ER medicine then talking about it would be important because it will help you get accross what types of things you imagine will be important for you as a physician and what your motivation to go to med school is. If you have experience in an ER and want to talk about it, mention the things you see in ER docs that makes you want to do that job, but make sure you are clear that your open to change your mind if you find that you can accomplish the same things in another specialty. In my case most of the things I like about pediatrics can be found in any primary care position, except the working with children part, and I'm pretty sure I mentioned that in my PS.
Also, some secondaries had a question about what specialties you were considering and why, so even if you don't mention it in your ps you could mention it elsewhere. I think med schools like that people have thought about where they see themselves in medicine even if they don't have a specific choice. Having a specialty (or several if that is the case) in mind shows you have given a lot of thought to why you want to be a physician and what you think you can contribute to the medical field, as long as you didn't just pick ER doc because you like the show...
so, to sum things up, talk about it if you can use your interest to explain who you are and what you want to do in medicine, but not if it is just some random preferance you came up with for no reason. And even if you feel sure that no matter what happens you want to work in an ER make sure you make it clear that you are willing to think it over again once you have a larger base of experiences to judge the different specialties from. You shouldn't let yourself get stuck in the mind set that you have to be an ER doc or you won't be happy, because you might end up an unhappy doc that way because you missed other opportunities.
Just my opinion there...hope it helps,
goofygirl