There is actually (what I think is) a very elegant and simple solution 
to the issue you bring up.  What we do to to explicitly define our 
constraints (we never use the schema generation stuff), which gives 
you the ability to name the constraint.  Then, when an exception does 
occur we (using the Spring framework) convert the jdbc exception to 
spring's DataAccessException.  If the exception happens to be 
something like a constraint violation, we then parse the name of the 
contraint violated and use the name in a resource bundle lookup for 
display to the end user.  Can't get much more user friendly than that; 
and I don't have to jump through any more hoops than necessary to do 
this vaidation thing.