I think you may have problems on Leopard if you have your default Java version set to 1.6.
Try running /Applications/Utilities/Java/Java Preferences.app. Check the order of the Java application versions. I have J2SE 5.0 on top.
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*
curl -o ~/Applications/arduino-0013/Arduino\ 13.app/Contents/Resources/Java/librxtxSerial.jnilib http://iharder.sourceforge.net/current/java/librxtxSerial.jnilib
Serial port '/dev/tty.usbserial-.....' already in use.
sudo mkdir /var/locksudo chmod 777 /var/lock