[XeTeX] Fractions using fontspec

[R (Chandra) Chandrasekhar]

Dear Folks,

I am using TeXLive 2008 from the DVD and kept updated regularly. I am 
trying to get fractions using the [Fractions=On] feature in fontspec. My 
minimal example file is:
---
\documentclass[a4paper,10pt]{article}
%
\usepackage{fontspec}
\usepackage{xltxtra}
\defaultfontfeatures{Mapping=tex-text,Scale=MatchLowercase}
\setmainfont[Numbers=OldStyle,Fractions=On]{Minion Pro}
\setsansfont[Numbers=OldStyle,Fractions=On]{Myriad Pro}
%
\begin{document}
This is one half: 1/2; this is one quarter: 1/4.

This is one half: \vfrac{1}{2}; this is one quarter: \vfrac{1}{4}.

\begin{sffamily}
  This is one half: 1/2; this is one quarter: 1/4.

This is one half: \vfrac{1}{2}; this is one quarter: \vfrac{1}{4}.
\end{sffamily}
\end{document}
---

The PDF output that I get from this file is attached and it is puzzling. 
I have several questions about it:

1. Why is the digit 1 in Lining style or uppercase in each of the 
plainly written fractions?

2. Why does the digit 4 in the \vfrac{} version look different from the 
digit in the plainly written fraction, but only in the roman family?

3. Is the a quirk of the version of the Adobe Minion Pro and Myriad Pro 
I am using?

4. What is the preferred method of getting fractions using solidus 
notation in XeLaTeX for OTF fonts? (It appears that the experimental 
xfrac package needs some coaxing; so \vfrac{} seems best to me at present.)

Thank you.

Chandra
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