Thanks for the suggestions mellis, I thought I'd try them out with the original file.
I think you may have problems on Leopard if you have your default Java version set to 1.6.
That's the explanation I've seen but I think there might be something different at play here.
Try running /Applications/Utilities/Java/Java Preferences.app. Check the order of the Java application versions. I have J2SE 5.0 on top.
My Java Preferences look like this--when it doesn't work:
If that doesn't work, trying right-clicking on Arduino.app and select "Show Package Contents". Edit Contents/Info.plist (with TextEdit) and change the JVMVersion from 1.4+ to 1.5*
Even with this, it still fails to work with the original library:
Odd. Exactly what version(s) of Mac OS X have you seen working?