In September 2011 I took out a 401K loan to assist in the down payment of my home. In January 2012 I switched my employer and do not plan to pay the loan back. My question is, when the loan goes into default, will I receive a 1099R for 2011 or 2012?

For 2012. The balance of the loan will be applied against the balance of the 401(k) and considered an in-service withdrawal. You will then have the balance added to your income as well as pay a 10% penalty.

And to add to what ScottGem said - because no income tax was withhled from your loan be prepared to owe income tax when you file your 2012 taxes next year. So be sure to save up enough cash to cover that bill. You should determine whether you will should make any estimated tax payments for 2012 to avoid interest charges from the IRS for underpayment of taxes.