Okay, everybody who says it's very looked down upon, check out my profile. Everybody always says that dropping out of grad school looks horrible. Well, it certainly doesn't look great, of course. But I happen to know a number of doctors who dropped out of grad school. I HATED being in my PhD program (the program itself was great; I just found out I couldn't stand the thought of actually being a scientist). I left grad school after my FIRST SEMESTER. Well, part-way into my second semester, actually, but they managed to remove all second semester courses from my transcript. And you know what? I got plenty of interviews and so far 3 acceptances and a waitlist (I'm waiting on another decision, to come in 2 weeks, and I'm hopeful for an acceptance). Grad school wasn't even brought up in all of my interviews! I addressed it briefly in my personal statement and moved on. It was probably held against me at some of the more prestigious schools, like Pitt and Penn and Emory, but I've still gotten into some good schools.
After leaving grad school last February I studied for and took the April MCAT and applied this past summer. I had a certain amount of clinical experience, but not a ton; I'm now working in a clinical job, which was something to discuss on interviews. Before I dropped out I met a woman who had dropped out of grad school after first semester and is now Chief of her ER. I also know a few other doctors who have done it.
Bottom line: it's completely doable. Don't stay somewhere if you're miserable. BUT you need to have good numbers, and some good ECs, to make it happen. And you need to address the whole issue succinctly, and not apologetically, in your application. I didn't stay any longer in grad school because an MS wasn't really an option in my program (except by default if you fail your prelim exam), and besides, I dreaded going in each day and I couldn't see myself doing that for any longer.
I have not regretted my decision for one day. In my current job I see patients, and every day I realize how wrong research was for me and how right medicine is. Interestingly, two other people dropped out of the same program (we started with 15) after I did; like I said, the program is great, but a life of research really takes a certain kind of person, and people aren't always aware of how it will fit until they get there. Above all, don't feel like you have to apologize to adcoms. It's your life, and you should do what works for you.
edit: sorry if the beginning of this post sounds harsh/defensive; it's a touchy subject, certainly very personal for me!