At Ceteva, we have put together a document which outlines a proposal for adding mechanisms to the Java language which allows it to be arbitrarily extended.
These mechanisms would support Language Oriented Programming (LOP) and Domain Specific Languages (DSLs). The proposal is based on some work we have been doing with XMF, an open source industrial strength language, and all the technology required to implement this mechanism in Java has been validated within XMF.
We are interested in receiving feedback on this proposal and any thoughts are most welcome. You can download the proposal document called "Beyond Annotations: A Proposal for Extensible Java (XJ)" from:
More details of XMF can be found on the same site.