Here is a post on their mailing list about it:

""" Since we bit the bullet and changed the on-disk format for JSONB data, the core committee feels that we should put out a new 9.4 beta release as soon as possible. Accordingly, we plan to wrap 9.4beta3 on Monday (Oct 6) for release Thursday Oct 9.

			regards, tom lane

"""

http://www.postgresql.org/message-id/[email protected]...