application.css not being served as an asset

EDIT 4, 5 and 6

8 hours in, any more ideas are welcome :) Maybe this bug is already known and solved, but I get the behaviour I described in edit 2&3, when you have this in a .css.erb file in app/assets/stylesheets :

li {
    background-image: url(<%= asset_path "logo_80px_80px.png" %>);
}

The problem part seems to be the asset_path method call, but .css.scss also breaks it with:

li {
    background-image: url(image-path("logo_80px_80px.png"));
}

My GemFile is:

source 'http://rubygems.org'

gem 'rails', '3.1.3'
gem 'pg', '0.11.0'
gem 'gravatar_image_tag', '1.0.0.pre2'
gem 'will_paginate'
gem 'rake'
gem 'jquery-rails'
gem 'nested_form'
gem 'acts-as-taggable-on'

gem 'sass-rails'
group :assets do
  gem 'coffee-rails', '~> 3.1.0'
  gem 'uglifier', '>= 1.0.3'
end

group :development do
  gem 'rspec-rails', '2.5.0'
  gem 'annotate', '2.4.0'
  gem 'faker', '0.3.1'
end


group :test do
  gem 'rspec', '2.5.0'
  gem 'webrat', '0.7.1'
  gem 'factory_girl_rails', '1.0'
end

group :production do
  gem 'therubyracer-heroku'
end

My Gemfile.lock is:

GEM
  remote: http://rubygems.org/
  specs:
    actionmailer (3.1.3)
      actionpack (= 3.1.3)
      mail (~> 2.3.0)
    actionpack (3.1.3)
      activemodel (= 3.1.3)
      activesupport (= 3.1.3)
      builder (~> 3.0.0)
      erubis (~> 2.7.0)
      i18n (~> 0.6)
      rack (~> 1.3.5)
      rack-cache (~> 1.1)
      rack-mount (~> 0.8.2)
      rack-test (~> 0.6.1)
      sprockets (~> 2.0.3)
    activemodel (3.1.3)
      activesupport (= 3.1.3)
      builder (~> 3.0.0)
      i18n (~> 0.6)
    activerecord (3.1.3)
      activemodel (= 3.1.3)
      activesupport (= 3.1.3)
      arel (~> 2.2.1)
      tzinfo (~> 0.3.29)
    activeresource (3.1.3)
      activemodel (= 3.1.3)
      activesupport (= 3.1.3)
    activesupport (3.1