Looks like, among other very interesting proposals, there is one about refining generics and breaking backwards compatibility:
Currently, generics are implemented
using erasure, which means that the
generic type information is not
available at runtime, which makes some
kind of code hard to write. Generics
were implemented this way to support
backwards compatibility with older
non-generic code. Reified generics
would make the generic type
information available at runtime,
which would break legacy non-generic
code. However, Neal Gafter has
proposed making types reifiable only
if specified, so as to not break
backward compatibility.
at Alex Miller's article about Java 7 Proposals
