After upgrading from OSX 10.2 to 10.3 (“Panther”), bold and italic fonts stopped working in JDK 1.4.1-based applications. The same applications running under JDK 1.3 (where such a test was possible) worked fine.
The problem (ferreted out by several developers on Apple’s javadev mailing list) lay with font files apparently installed by Microsoft Office. Fortunately, the offending fonts were not installed on top of the “correct” fonts, so the fix is simple. Each user on the system (potentially) has a Library/Fonts/ directory within their home directory (i.e., ~/Library/Fonts/). There is also a system font directory, inside the root-level /Library/ directory on the boot disk (/Library/Fonts/). Deleting or, to be safer, moving, any fonts that have files in both directories solves the problem.
On my machine, the Arial, Arial Black, Times New Roman, and Verdana fonts were duplicated. Being paranoid, I moved these to another directory until I was sure that Office still worked correctly after the change. I also found that I had to log out and back in. Others have evidently had different experiences, including a reported need to reboot to make the changes take effect.