This report from the Kaiser Family Foundation explores challenges that may arise in eligibility determination between state exchanges and Medicaid. Medicaid eligibility is determined by current income level, whereas insurance exchanges will likely use past tax returns to determine eligibilty for tax credits.

From the report:

A major goal of the Affordable Care Act (ACA) is to significantly expand coverage and reduce the number of uninsured. Beginning in 2014, the ACA establishes a new continuum of coverage that will provide assistance to people with incomes up to 400% of the federal poverty level. Under this expansion, Medicaid eligibility will be expanded to a national floor of 138% of poverty and premium tax credits will be available to eligible individuals with incomes between 139% and 400% of poverty to purchase coverage through new Health Insurance Exchanges.

Full Report: Determining Income for Adults Applying for Medicaid and Exchange Coverage Subsidies (PDF | 575 KB) 

Kaiser Family Foundation. (2011). Determining income for adults applying for Medicaid and exchange coverage subsidies.