Hi,
Solved this one. It was a matter of removing some hibernate 3.6 jars which were apparently conflicting with the hibernate-tools plugin's own jars.
Here's what you need to do to succesfully reverse engineer the database tables without getting the errors i got:
you must have in the project build path only the hibernate3.jar and the jdbc driver ( in my case postgresql-9.0-blabla.jar), the others (antlr, dom4j, slf4j, jta,etc.) should be removed; i have not tried to reverse-engineer in JPA mode but i reckon you'll have to keep the hinernate-jpa-2.0-api-blabla.jar in your project's build path.
After you finished using hibernate-tools plugin you must include the other jars because otherwise the hibernate code won't work ( i think, i haven't tested yet, but if the documentation for 3.6 says so... ) .
If anyone has anything to add please take the time and do it, as i have.
Thanks