You're right, ofcourse, but it might be possible to re-use parts of GNU classpath to work with an alternative VM/interpreter. IIRC there are several free java projects (including Harmony) that (plan to) draw from it. I haven't looked at classpath in any detail, but it seems to me that the parts written in Java should at least in theory be portable.
True, but it's the only free implementation that I know of that has at least some part done (and looks like it might get somewhere). As you said, the Java standard library is huge.