This article on OnJava.com demonstrates the use of the HibernateSynchroniser plugin for Eclipse that apparently keeps hbm mappings in line with Java code.
The page is a bit old but references Eclipse 3.2, so hopefully the plugin is still useful.
I’ve been using the JBoss Hibernate Tools which just entered 3.2 GA and it seems to be making leaps and bounds. The only crux is that you need to do a lot of preconfig work, it may be harder to introduce into an already established project that does its hibernate config programmatically.