[XeTeX] Diacritic marks in various fonts

[nathan.sidoli at utoronto.ca]

I'm having some issues with the dot diacritic under m and n, \d{m} and  
\d{n} or ? and ?, with italicized typeface in some fonts, particularly  
Junicode.

Whenever I use Junicode or Times Translit, I get blank spaces for  
these terms, and a warning message such as the following:

LaTeX Font Warning: Font shape `OT1/cmr/m/n' in size <8.5> not available
(Font)              size <8> substituted on input line 75.


When I use Times, New Times Roman, Gentium Plus, etc. I get the same  
warning, but the characters are displaying properly. I suppose this is  
a problem with the fonts themselves, but I'm wondering if anyone has a  
suggested work-around.

Thanks,

Nathan