Just reinforcing @Jay Riggs answer, and because I don't have enough reputation to just comment, I would go with:
String.Format(numberInfo, "{0:p0}", 0.10);
I think you are covering situations where you need to format more than one value:
String.Format(numberInfo, "{0:p0} {1:p0}", 0.10, 0.20);
Vladimir 03 Jun. '11 at 20:01 2011-06-03 20:01
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