Personally, I'd be happy with something like this:
Jython 2.2: Java 1.2
Jython 2.3: Java 1.4
Jython 2.4: Java 1.4
but moving to 1.5 compatibility pretty much means Jython cannot be
used in most enterprises. Enterprise IT departments tend to be VERY
conservative (at least when it comes to "infrastructure" like the
JVM), Java 1.4.2 is pretty much the norm at almost all companies that
I've been involved with recently, and that conservativeness shows
little sign of changing anytime soon.
Jython 3.0 may be a good opportunity to take the leap to a modern JVM
however, since then we will have an actively maintained and up-to-
date 2.x version for people to use.