[XeTeX] Diacritical accent marks using \.{}, \d{} in Tamil/Malayalam

[P. P. Narayanaswami]

Thanks.  I get the dots above/below, if I load xunicode package "before"
fontspec. If I use it after the fontspec, nothing happens.  Also, the 
dots are not centered properly above or below the characters.
The dot below using \d{} is somewhat satisfactory, but the dot above
using \.{} is way off the character too much shifted to left/right.

Here is the test file:
\documentclass{article}
\usepackage{xunicode}
\usepackage{fontspec}
\def\tam{\fontspec[Script=Tamil]{Latha}}  
\begin{document}
\tam 
\d{{ரி}}  \.{{ரி}}
\end{document}