Per diem income is sort of like self-employment income when it comes to calculating the monthly income, used to figure out your debt to income ratio.
This means the income will be averaged for the past two years (if 2011 income is bigger or same as 2010). The Income Verification form will be sent to your employer - to see that 2012 YTD is within the average income for 2010-2011.
Hope this helps,
Beachfront Realty, Inc.