With PostgreSQL 9.2, is there a way to determine the amount of space considered available within the database? More specifically, the approximate amount of space that can be used before the disk space currently associated with the DB will increase (as reported by pg_database_size()).

Imagine having a small DB, say 10gb, where a user has removed 1/2 of the entries. Even after a vacuum has been run, space has not been returned to the file system (due to space constraints a vacuum full is not a possibility). What I'd like to understand is approximately how much space is now considered 'available' after 1/2 of the entries were removed.