It's currently possible to download an archive of a specific revision by commit id (e.g. http://bitbucket.org/user/project/get/123456789abc.zip).
It would be nice if this would also work for tags (e.g. http://bitbucket.org/user/project/get/v0.9.zip).
Btw: My I suggest to name tarballs .tar.gz (instead of just .gz) so they are handled properly by file managers.