Thanks.
Actually I've found what I was looking for. There seem to be some
subtle differences in the bytecode format a la:

--
As of J2SE 1.4.0, the javac bytecode compiler uses "-target 1.2" by
default as opposed to the previous "-target 1.1" behavior. See the
reference page for the javac compiler for descriptions of these
behaviors. One of the changes involved in targeting 1.2 is that the
compiler no longer generates and inserts method declarations into
class files when the class inherits unimplemented methods from
interfaces. These inserted methods, like all other non-private
methods, are included in the default serialVersionUID computation. As
a result, if you define an abstract serializable class which directly
implements an interface but does not implement one or more of its
methods, then its default serialVersionUID value will vary depending
on whether it is compiled with the J2SE 1.4 version of javac or a
previous javac.
--

and I guess that coupled with the slightly sketchy Waba VM, that's
enough to stop it working.