The one aspect that often "trips people up" is the IRS requirement that you must aggregate all your IRAs to determine tax liability, Mayabb says. "You have to consider all your IRAs and where those dollars came from," she says. "If you have another IRA out there and if you originally took a tax deduction on those dollars, that will affect your tax liability on the Roth IRA."
http://www.bankrate.com/finance/retirement/drawback-one-type-roth-conversion.aspx