Several posts have mentioned the idea of a change in rules that will result in taxation. My assumption is that the thinking is that this would be on the gains, since you've already paid taxes on the contribution/conversion.
From a practical standpoint, this would be tough to do, since there is no current requirement folks need to have the information on what the "basis" is. You think the financial firm has it? Not necessarily... One of my kids has had a Roth for about 15 years. I started it when he was 18 and had some earned income while going to college. I continued to contribute while he was in college, and he did for the first few years after. Fast forward to today, and that account has grown nicely. He decided to look into getting the contributions out to invest in real estate (he's in CA and has made a great return over the past 6 years on his house, personally I think it's a dumb idea - voluntarily moving tax free $ to taxable...
). Upon contacting Fidelity, who holds the Roth, they indicated they don't have info for the life of the account, I think they went back 5 years. Of course, maybe it's there but tough to get, or you have to pay a fee to get old statements, etc. I didn't have all the info since I'd purged tax returns and account statements when I moved. In hindsight, I should have maintained the info, but I didn't think about the possibility that this could arise.
If anything was implemented, I'd have to believe it would be going forward. Saying you have to pay tax on ALL of it would require double taxation, while only the gains is a number that may be difficult to impossible to get your hands on it. Another possibility is an indirect way to tax, like a VAT.
Speculation as always, but I'm doing conversions without any expectation that rules could change retroactively.