As of this writing (9 Jan 2010), we do not make special release versions of Backbone, because the main source tree should always be in what some other projects would call 'release condition'.
If you would like to try Backbone, first you need a copy of the Git version control system. Once you have Git working, you can get a copy of Backbone using the following incantation:
git clone git://git.savannah.nongnu.org/backbone.git
Watch this space in the future for more information. :)