Hmm . . . this is an interesting situation, as I find myself wondering why you would have applied to grad programs in design if you had already made the decision to leave the field. It sounds to me as though you might still be interested in sticking with a career in design, as I'm not sure why you'd go through a grad program (with loans?) if it gets you nowhere other than 2 years further down the road with some skills that won't directly translate to your ultimate career in medicine. (I didn't find grad school to be a picnic, and I can imagine your design studios will be quite intense). If you are still thinking you might want to stay in design, I would say to do the grad program, as it can only help further your design career. Another option: can you defer your grad school acceptance for a year while you start the post-bacc program, do some more medical volunteering and some physician shadowing? That way, if you get into the nitty gritty of science classes, shadowing, etc. and you change your mind about the whole thing, you can pick up where you left off with your design career, secure in the knowledge that you've left no stone unturned.
If you're sure about med school, then why wait? If it's just to get that piece of paper from Harvard, I'd think long and hard about it. The Harvard name is always impressive, of course . . . but when it comes to med school apps, I've seen on here so many times that grad school - particularly in a non-science field - is basically considered a "nice extracurricular" and nothing more. (I did a grad program in art history at a prestigious school, and I think that was definitely true in my case.) The grad GPA will appear on your apps, but will be completely separate from any ugrad and post-bacc grades (the ones that really matter), so I have a feeling it's just glanced at to make sure you didn't completely flunk out of grad school.
One word of caution: if you do do the grad school route and then change your mind midway though, I'd finish it out (1.5 years isn't too big a deal). Med schools don't particularly like to see that you've dropped out of a grad program. Also, if you go the grad route and think you might still want to go to med school, keep volunteering and try to do some shadowing. Med schools like to see altruism and dedication to a cause, and continuing to be in a medical environment should help you think about whether you want to continue down that path.
In any case, best of luck to you!
