The comments in sun.xxx.Ref say:

... When space gets tight, the garbage collector
will clear old Ref cache entries when there are no other pointers to the
object. ...
 
But the marking code won't create a hard mark on an object if it is already soft marked.

The folks doing RMI here are using Refs extensively to implement the distributed
GC you need for distributed objects. Let me know if this won't get fixed, and I'll
pass this on to them.