qariwa, we must be doing something wrong - I have tried with other books, and fbreader works perfectly well at changing fonts. I cannot see what the problem is, as I can read the same book on the PB360 in the font I like, so I would have thought there shouldn'd be a major problem with the book itself, but obviously there is. And it was indeed strange that nobody was complaining about that.
EDIT: problem solved (at least in my case). My EPUB are all translated, and in the CSS style file for the declaration of the text I had:
font-family: "LexiaDaMa", serif;
The font family declaration (highlighted in bold) for some reason seem to be creating the problems in the PB903 - I removed it, and all is well. Hope it works for you too!