According to a part of the OpenType standard at the following address: http://www.microsoft.com/typography/otspec/os2.htm#fci The usFirstCharIndex value should contain "The minimum Unicode index (character code) in this font, according to the cmap subtable for platform ID 3 and platform- specific encoding ID 0 or 1." This is not counting the value 0, which is the ".notdef" character. This means that the value should be 0x20 for all the DejaVu fonts, as space is the first character supported. But the value in the fonts, according to TTX and also MS font validator is 0x0d (13 decimal).
I think FontForge is automatically putting that value there, because I can't find a place to set that value. Going to forward this to the FontForge mailing list.
George Williams has patched this in FontForge, should work now with latest svn of FF.
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