Technically you are in default after 30 days of missing your first payment.
Most lenders will not start the foreclosure process until you have missed 3 payments but they have the right if they choose to start hte procedure sooner.
A quick call to your lender will give you the answer of how they would treat your particular case.
The banks would prefer you to continue to pay the orginal terms of the mortgage rather than foreclose on you since they have to recognize a loss on their books if they foreclose you.
This can affect their liquidity and solvency so they will do everything they can to modify your loan and keep you as a revenue stream.
Given the soft housing market this is especially true right now in 2010.