Backwards compatability is always a difficult balance. Each version of Java makes changes to the language and also (perhaps more importantly) fixes issues in the JVM. For example we saw lots of CMM color issues fixed in Java 6! Some customers need to support legacy code or wait for dependencies – it often takes some time for JBoss, Websphere and other big code bases to move onto a new version of Java.
But Java is evolving (and Oracle now has a clear development path for it). If we continued to support Java 1.3 (or maybe 1.1!) our customers would not be able to benefit from improvements in Java. Oracle makes Java 6 the minimum specification for many things such as JavaFX.
Oracle has now confirmed that Java 6 support will end in February 2013 (see https://blogs.oracle.com/java/entry/end_of_public_updates_for). It came out in 2006 while Java 5 was released in 2004. We will also see Java 8 in 2013.
So we are proposing to make Java 6 our minimum requirement from January 2013. What do you think?