You cannot deploy Java 6 applications to Mac (except 64-bit Intel Mac OS X 10.5 after September, as mentioned above). You can build Java 6 for all other platforms (back to Windows 98, FFS), but to work for Mac customers you have to adjust your build to create a Java 1.5 version.
You also have to add in special Java upgrade instructions for those using OS X 10.4 (since the 1.5 upgrade isn't included in automatic updates), and tell anyone with a Mac earlier than that to piss off, because Java 1.5 (released over FOUR years ago, in 2004) won't work on OS X 10.3 or earlier.
That is an exceptional pain which isn't present on Windows or Linux.
Apple makes wonderful hardware but they treat their third-party developers like crap. Microsoft is absolutely awesome in comparison and realizes that "Developers, Developers, Developers" are the secret to their OS's success.