I get a different error, using TL 2010 pre-release :
(e:/TeX/Live/2010/texmf-dist/tex/latex/fontspec/fontspec.cfg))
! Undefined control sequence.
<recently read> \newfontinstance
l.3 \newfontinstance
\dhivehifontA{Mv Elaaf Normal}
Philip Taylor
--------
\documentclass[12pt,letterpaper]{report}
\usepackage{fontspec}
\newfontinstance\dhivehifontA{Mv Elaaf Normal}
\newcommand{\dhiA}[1]{{\newfontinstance\dhivehifontA #1}}
\begin{document}
\dhiA{ޓީވީއެމް އާއި ދިވެހިރާއްޖޭގެ އަޑުގެ ނަން ބަދަލުކޮށްފިއެވެ.}
\end{document}
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