Heroku, Rails 3.2.11, Assets.

Em config/application.rb:

config.assets.precompile += %w( *.css *.js)
config.assets.initialize_on_precompile = false

Em config/environments/production.rb:

config.serve_static_assets = true
config.static_cache_control = "public, max-age=3153600"
config.assets.compress = true
config.assets.compile = false
config.assets.digest = true

(liste seus aplicativos Heroku):

heroku apps

Habilite o plugin ( doc aqui ), em seu shell:

heroku labs:enable user-env-compile -a my_heroku_app

commit, push:

git add .
git commit -m 'assets install'
git push heroku master

Obrigado @blakink