I'm doing a similar thing and applying this year. I'm in my 5th year though, and I'm almost positive I'll be finished in time to matriculate.
As for quitting in the middle and applying, I had thought about doing that, but decided it really wasn't a good idea b/c most schools will see one thing: you quit. They'll be scared a similar thing will happen in med school.
If you are towards the very beginning of your PhD, you can consider switching to a MD/PhD. I know a couple of people who did that, but again, it's looked down on if you're too far along. I know, at least at my school, they are wary of the students just trying use the system to get their med school paid for.
Personally, I would stick it out a couple more years... the final years of your PhD are actually the easiest since it's really nothing but research and writing. Get as many papers published as you can, study hard for the MCAT, and then apply in a couple of years when you are almost done... at least that's what I did.