Yeah, they could totally shut off conversions or limit them in a way similar to pre-2010 (I think), but the proposals that the White House issued this time last year basically called for eliminating the ability to convert any money that isn't "taxable" into Roth. So "pre-tax" IRA/401k money is taxable on conversion and therefore allowed; after-tax "non-deductible" tIRA money could not be converted, which is the backdoor Roth.