I don't think you can download the source to the java virtual machine--just the java classes. If we want JDK1.4 (still beta anyway) we'll have to wait for Apple to make it, since Sun only puts out Intel and Sparc versions of the JDK.
Yeah, if you want the new XML support and such, you could probably just copy an rt.jar file from windows, but you never know if they did anything else funny to it.