Ask yourself what you want to do. If you really want to be a doctor, you would more likely be asking about working in a clinical environment than I-banking. I doubt it will hurt you at all in the admissions cycle if you are genuinely interested in medicine (and you may even be able to save up for medical school). But since you had to ask this I would recommend asking yourself if you really want to do medicine. As for the people I know who were successful in the admissions cycle, this question would not have crossed their minds. They were volunteering full-time in the clinic in their year-off, not doing "I-banking".
If you go down the medicine pathway, you will have sooooo much time for 80 hour weeks. Do you really want to spend your entire prime doing that? There are cooler things to do in your year off (that are a lot less lucrative but more beneficial to your career, to you as a person, and to your skills) like working in Americorps, volunteering in Central America to learn Spanish, or doing research. I'd recommend those instead. But obviously, it's up to you to make this decision.
Edit: I'd be skeptical of the economic benefit of working for a year in I-banking and then going to medical school versus going straight in and earning an attending's salary a year earlier. Including interest/inflation/yadda yadda the argument of "I wanted to save for medical school" might not even make sense. You're the I-banking guy so I'm sure you can figure this one out. 😉