At this point, I thought 1.4.1 wasn't the problem, but a clean install of JBuilder after the update still wouldn't work. Knowing that JBuilder is simply a HUGE Java app anyhow, I decided to run the class from the command prompt. Here's the long classpath that gets this behemoth up and running:
java -cp /System/Library/Frameworks/JavaVM.framework/Versions/1.4.1/Classes/classes.jar:
../patch:../lib:../lib/ext:../lib/TabbedPaneFix.jar:../lib/lawt.jar:../lib/jbuilder.jar:
../lib/gnuregexp.jar:../lib/help.jar:../doc/jb_ui.jar:../lib/activation.jar:../lib/ant.jar:
../lib/beandt.jar:../lib/dt.jar:../lib/dx.jar:../lib/jbcl.jar:../lib/optional.jar:
../lib/xerces.jar com.borland.jbuilder.JBuilder
**Note that everything above should be all on one line with no spaces between the class path items.
[robg adds: I don't have JBuilder, so I can't test this, and I hope I didn't mess up the command while formatting it for display. Please let me know if I did!]

