> My questions are then:
> - is there any differences between the code sources of the different
> binaries JDK 1.3, JDK 1.4, JDK 1.5, JDK 1.6?
> - if yes, what are they? and what should we be aware when choosing?
> - if no, I suppose then the only difference is to compile with target
> 1.x, then why doing so?

Mark answered your question already. The reason is to compile the jars
on different platforms. I am not sure if I get Jaybird for Java 6
running on Java 5, but I can check.

Roman