It appears that Roth IRAs are subject to the mandatory withholding requirements of regular IRAs. Is this true? If not, please provide cite. If it is true, do we withhold on the full amount of the distribution (i.e., including those portions aready taxable to the partiicpant) or only on the taxable portion? Cite would be appreciated.

Withholding for the Roth IRA is the same as that for the traditional IRA. Check proposed regulation section 1.408A-6, Q&A-12, 13. Withholding would be on the entire amount of the distribution, just like it would be on a distribution from a traditional IRA.