[XeTeX] Devanagari ligature issue

[Guy]

Greetings all,

I've just begun to use XeTeX on linux (ubuntu, via TeXLive) and
haven't been able to get Devanagari to appear correctly.  The output
essentially undoes the ligatures formed by my input method editor
(scim / m17n), leaving a mess of viraama-s and mis-sequenced short
'i'-s.   (I've tried several unicode fonts which work well with other
unicode-specific programs (Sanskrit 2003, Uttara, Santipur OT), as
well as different unicode-capable editors, but all yield the same
problem.  Is there anything specific to naagari typesetting which
needs to be specified in the preamble?  Any help would be much
appreciated.

best,
Guy