Thanks Bob. I'd actually just come across that article myself while looking through the code.
I was trying to find the rule -- which I'm sure I've seen -- that in a residential system no conductor must be more than 150V to ground. I was thinking that any xfmr used for 220V output would need a center-tap ground to satisfy that requirement.
Still 210.6 kind of makes that academic now anyway. I'll break the bad news.
Thanks.