Jumping before you're ready runs the risk of losing it all over a relatively small hitch in the plans, then you lose the house and don't have enough for a security deposit on an apartment.
I don't think that's how it works. When a bank forecloses on your house you are normally allowed to live there mortgage free. Amazingly the foreclosure process often takes from 1 to 2 years. Which should give you plenty of time to save for an apartment security deposit. Some times the bank will allow you to short sell and will not come after you for the amount you still owe. Go figure!
I think the down payment is for the banks protection, not the home owners. Don't get me wrong. I think banks should require a 20% down payment. But, financially speaking, the home owner should be more concerned about whether he's getting the house for a good price and whether he can afford the monthly payment (which includes pmi), utilities, etc. The bank should be concerned about the down payment.