This week's book giveaway is in the Agile and Other Processes forum. We're giving away four copies of The Little Book of Impediments (e-book only) and have Tom Perry on-line! See this thread for details.
The possible locations of the persistence.xml is pre-defined by the JPA spec. But the user application does not need to know the location of the persistence.xml. It's upto the persistence provider (like Hibernate) to search and find the persistence.xml in appropriate locations.