Whether the bank in question will foreclose on you is at the discretion of the lender.
The normal process is to issue you a notice of default after 30 days.
Most lenders will give you 3 missed payments (or 90 days)
depending on your state before they will initiate foreclosure proceedings.
Having said that banks in the US are reluctant to foreclose under current market conditions because they would have to book the loss if they foreclose.
This is allowing a lot of US home owners to stay in their homes without paying their mortgages for an extended period before banks are foreclosing.
Banks are hoping this down term in housing will end in the near future and make these underwater mortgages whole.
It`s a gamble the banks are making that housing will recover but if I was in your shoes I would not consider stopping to pay your mortgage.