Piggyback loans like the 80/15/5 aren't just for first time buyers -- in fact, that is my favorite loan, and I have used it for my past few houses.
For most "conforming" loans (fully documented, excellent credit) a debt:income ratio of 1:2 is what most lenders need, at least in my experience.
I think that many of the sub-prime loans (including 100% loans) are going to be much harder to get.
Unless you are borrowing a LOT of money in Med. school and plan on making very little, I don't think you will have a problem obtaining a conforming loan ever. To my knowledge, there haven't really been any significant changes to that market since piggyback loans and ARMs. The loan qual. requirements loosened a bit (I seem to remember for my first house, I needed a debt:income ratio of no more than 36%, but now it is up to 50%) but I don't see a big shakeup in that market. Buying the house in residency with no down payment might be a problem though, as, like I said before, I would expect 100% loans to be harder to get and/or more expensive.