From REW's link: this may be relevant.........
If you made nondeductible contributions to a traditional IRA at any time in the past, and haven't previously withdrawn the nondeductible contributions, then your partial conversion will be partly nontaxable. The rules that apply here are the same as for any distribution from a traditional IRA. You treat all traditional IRAs as a single IRA, so you don't get a different result depending on which IRA you convert. You add up the total value of all traditional IRAs and also the total amount of nondeductible contributions to those IRAs to determine what percentage of the conversion amount is taxable. For example, if 60% of your IRA balance comes from nondeductible contributions and you convert $8,000 of that IRA, you'll report $3,200 of income from the conversion (40% of $8,000).
It would be nice if you could specify that you're converting only the nontaxable portion. The rules don't let you do that.