Problem with embedded fonts.
In a test epup I use three FreeSerif...ttf (regular, bold and italic). If I use <b>... </b> und <i>... </i> for bold and italic Sigil shows the text only in regular. But the Sony PRS-300 shows it correct, it means the bold and italic fonts are used, not the modifikations of regular.
http://c0f.de/down/_template_mit_freeserif.epub