It appears you’re using fonts from the custom design upgrade? As far as I know they don’t have any browser conflicts with that, but since wordpress.com sells it and it’s not bundled in the theme, you might want to ask them about it.
A good test would be to disable your customizations and see if the font works correctly.
OK, weirdest thing. Since this seemed like a CSS issue, I went back to the custom CSS field. I had added a line earlier that didn’t work out, so I’d deleted that (before any issue started). On a whim, I added a new blank line after the comment code that appears there by default and… now the site looks fine.
So maybe some invisible garbage character was showing up there or something? Anyway, fixed now. Thanks CSS Gremlins!