I ran into this the other day too. Here's my guess: Fonts have different character sets for italic and bold (in some cases they're completely different font files). In previous versions of Flash it created fake bold and italic characters, simply taking your font and making the letter thicker, or skewing it. In CS5 they decided to stop doing that, and instead use the "real" fonts. Problem is if you don't have the "real" bold font installed, you have no option to use it.