Hi,
I know the following link is not official (not much official documentation exists other that an excerpt found in the first link) but it is helpfull (also check out the blog mentioned in that thread),
http://stackoverflow.com/questions/438146/hibernate-question-hbm2ddl-auto-possible-values-and-what-they-do
However, whatever your decision regarding hibernate.hbm2ddl.auto , take a look at the following opinion on this matter in production environments,
http://stackoverflow.com/questions/221379/hibernate-hbm2ddl-auto-update-in-production